Move Chisel3 to SPDX license conventions (#1604)

Change source and other relevant files to use SPDX license
LICENSE file moved from src/ to ./
Changed license file to refer to this per recommendation
using_spdx_license_list_short_identifiers

WARNING: Tests fail with as of yet undiagnosed error
```
[error] Failed: Total 691, Failed 19, Errors 0, Passed 672, Ignored 15
[error] Failed tests:
[error] 	chiselTests.QueueSpec
[error] 	examples.VendingMachineGeneratorSpec
[error] 	chiselTests.HarnessSpec
[error] 	chiselTests.ConnectSpec
[error] 	chiselTests.aop.SelectSpec
[error] 	chiselTests.PopCountSpec
[error] 	chiselTests.CloneModuleSpec
[error] (Test / test) sbt.TestsFailedException: Tests unsuccessful
[error] Total time: 379 s (06:19), completed Sep 30, 2020 12:38:17 AM
sbt:chisel3>
```

28 files changed, 28 insertions, 28 deletions

diff --git a/src/main/scala/chisel3/util/Arbiter.scala b/src/main/scala/chisel3/util/Arbiter.scala
index 056279e0..059bdd14 100644
--- a/src/main/scala/chisel3/util/Arbiter.scala
+++ b/src/main/scala/chisel3/util/Arbiter.scala
@@ -1,4 +1,4 @@
-// See LICENSE for license details.
+// SPDX-License-Identifier: Apache-2.0
 
 /** Arbiters in all shapes and sizes.
   */
diff --git a/src/main/scala/chisel3/util/BitPat.scala b/src/main/scala/chisel3/util/BitPat.scala
index 27de9982..40563e23 100644
--- a/src/main/scala/chisel3/util/BitPat.scala
+++ b/src/main/scala/chisel3/util/BitPat.scala
@@ -1,4 +1,4 @@
-// See LICENSE for license details.
+// SPDX-License-Identifier: Apache-2.0
 
 package chisel3.util
 
diff --git a/src/main/scala/chisel3/util/Bitwise.scala b/src/main/scala/chisel3/util/Bitwise.scala
index 3583b7d1..ddf7546a 100644
--- a/src/main/scala/chisel3/util/Bitwise.scala
+++ b/src/main/scala/chisel3/util/Bitwise.scala
@@ -1,4 +1,4 @@
-// See LICENSE for license details.
+// SPDX-License-Identifier: Apache-2.0
 
 /** Miscellaneous circuit generators operating on bits.
   */
diff --git a/src/main/scala/chisel3/util/BlackBoxUtils.scala b/src/main/scala/chisel3/util/BlackBoxUtils.scala
index 7f9c117f..74d99ff8 100644
--- a/src/main/scala/chisel3/util/BlackBoxUtils.scala
+++ b/src/main/scala/chisel3/util/BlackBoxUtils.scala
@@ -1,4 +1,4 @@
-// See LICENSE for license details.
+// SPDX-License-Identifier: Apache-2.0
 
 package chisel3.util
 
diff --git a/src/main/scala/chisel3/util/Cat.scala b/src/main/scala/chisel3/util/Cat.scala
index ee01c6e6..793a5463 100644
--- a/src/main/scala/chisel3/util/Cat.scala
+++ b/src/main/scala/chisel3/util/Cat.scala
@@ -1,4 +1,4 @@
-// See LICENSE for license details.
+// SPDX-License-Identifier: Apache-2.0
 
 package chisel3.util
 
diff --git a/src/main/scala/chisel3/util/CircuitMath.scala b/src/main/scala/chisel3/util/CircuitMath.scala
index 53d47694..58961630 100644
--- a/src/main/scala/chisel3/util/CircuitMath.scala
+++ b/src/main/scala/chisel3/util/CircuitMath.scala
@@ -1,4 +1,4 @@
-// See LICENSE for license details.
+// SPDX-License-Identifier: Apache-2.0
 
 /** Circuit-land math operations.
   */
diff --git a/src/main/scala/chisel3/util/Conditional.scala b/src/main/scala/chisel3/util/Conditional.scala
index e9d216f5..ca393055 100644
--- a/src/main/scala/chisel3/util/Conditional.scala
+++ b/src/main/scala/chisel3/util/Conditional.scala
@@ -1,4 +1,4 @@
-// See LICENSE for license details.
+// SPDX-License-Identifier: Apache-2.0
 
 /** Conditional blocks.
   */
diff --git a/src/main/scala/chisel3/util/Counter.scala b/src/main/scala/chisel3/util/Counter.scala
index 603ebeb6..5910c703 100644
--- a/src/main/scala/chisel3/util/Counter.scala
+++ b/src/main/scala/chisel3/util/Counter.scala
@@ -1,4 +1,4 @@
-// See LICENSE for license details.
+// SPDX-License-Identifier: Apache-2.0
 
 package chisel3.util
 
diff --git a/src/main/scala/chisel3/util/Decoupled.scala b/src/main/scala/chisel3/util/Decoupled.scala
index ade93717..c6459455 100644
--- a/src/main/scala/chisel3/util/Decoupled.scala
+++ b/src/main/scala/chisel3/util/Decoupled.scala
@@ -1,4 +1,4 @@
-// See LICENSE for license details.
+// SPDX-License-Identifier: Apache-2.0
 
 /** Wrappers for ready-valid (Decoupled) interfaces and associated circuit generators using them.
   */
diff --git a/src/main/scala/chisel3/util/Enum.scala b/src/main/scala/chisel3/util/Enum.scala
index 3fb9f9e4..bf150464 100644
--- a/src/main/scala/chisel3/util/Enum.scala
+++ b/src/main/scala/chisel3/util/Enum.scala
@@ -1,4 +1,4 @@
-// See LICENSE for license details.
+// SPDX-License-Identifier: Apache-2.0
 
 /** Enum generators, allowing circuit constants to have more meaningful names.
   */
diff --git a/src/main/scala/chisel3/util/ImplicitConversions.scala b/src/main/scala/chisel3/util/ImplicitConversions.scala
index 4c89acdd..792280ac 100644
--- a/src/main/scala/chisel3/util/ImplicitConversions.scala
+++ b/src/main/scala/chisel3/util/ImplicitConversions.scala
@@ -1,4 +1,4 @@
-// See LICENSE for license details.
+// SPDX-License-Identifier: Apache-2.0
 
 package chisel3.util
 
diff --git a/src/main/scala/chisel3/util/Lookup.scala b/src/main/scala/chisel3/util/Lookup.scala
index 1152bdb4..e0ed694f 100644
--- a/src/main/scala/chisel3/util/Lookup.scala
+++ b/src/main/scala/chisel3/util/Lookup.scala
@@ -1,4 +1,4 @@
-// See LICENSE for license details.
+// SPDX-License-Identifier: Apache-2.0
 
 package chisel3.util
 
diff --git a/src/main/scala/chisel3/util/Math.scala b/src/main/scala/chisel3/util/Math.scala
index 105b4161..2a833e80 100644
--- a/src/main/scala/chisel3/util/Math.scala
+++ b/src/main/scala/chisel3/util/Math.scala
@@ -1,4 +1,4 @@
-// See LICENSE for license details.
+// SPDX-License-Identifier: Apache-2.0
 
 /** Scala-land math helper functions, like logs.
   */
diff --git a/src/main/scala/chisel3/util/MixedVec.scala b/src/main/scala/chisel3/util/MixedVec.scala
index 70b0656f..a632ec3a 100644
--- a/src/main/scala/chisel3/util/MixedVec.scala
+++ b/src/main/scala/chisel3/util/MixedVec.scala
@@ -1,4 +1,4 @@
-// See LICENSE for license details.
+// SPDX-License-Identifier: Apache-2.0
 
 package chisel3.util
 
diff --git a/src/main/scala/chisel3/util/Mux.scala b/src/main/scala/chisel3/util/Mux.scala
index ea6bf6a1..946de461 100644
--- a/src/main/scala/chisel3/util/Mux.scala
+++ b/src/main/scala/chisel3/util/Mux.scala
@@ -1,4 +1,4 @@
-// See LICENSE for license details.
+// SPDX-License-Identifier: Apache-2.0
 
 /** Mux circuit generators.
   */
diff --git a/src/main/scala/chisel3/util/OneHot.scala b/src/main/scala/chisel3/util/OneHot.scala
index 776e3f6f..03f470c7 100644
--- a/src/main/scala/chisel3/util/OneHot.scala
+++ b/src/main/scala/chisel3/util/OneHot.scala
@@ -1,4 +1,4 @@
-// See LICENSE for license details.
+// SPDX-License-Identifier: Apache-2.0
 
 /** Circuit generators for working with one-hot representations.
   */
diff --git a/src/main/scala/chisel3/util/Reg.scala b/src/main/scala/chisel3/util/Reg.scala
index 2101f998..982d80d0 100644
--- a/src/main/scala/chisel3/util/Reg.scala
+++ b/src/main/scala/chisel3/util/Reg.scala
@@ -1,4 +1,4 @@
-// See LICENSE for license details.
+// SPDX-License-Identifier: Apache-2.0
 
 package chisel3.util
 
diff --git a/src/main/scala/chisel3/util/TransitName.scala b/src/main/scala/chisel3/util/TransitName.scala
index 992f48d2..97c13696 100644
--- a/src/main/scala/chisel3/util/TransitName.scala
+++ b/src/main/scala/chisel3/util/TransitName.scala
@@ -1,4 +1,4 @@
-// See LICENSE for license details.
+// SPDX-License-Identifier: Apache-2.0
 
 package chisel3.util
 
diff --git a/src/main/scala/chisel3/util/Valid.scala b/src/main/scala/chisel3/util/Valid.scala
index 1bd398f2..6c6d685e 100644
--- a/src/main/scala/chisel3/util/Valid.scala
+++ b/src/main/scala/chisel3/util/Valid.scala
@@ -1,4 +1,4 @@
-// See LICENSE for license details.
+// SPDX-License-Identifier: Apache-2.0
 
 /** Wrappers for valid interfaces and associated circuit generators using them.
   */
diff --git a/src/main/scala/chisel3/util/experimental/BoringUtils.scala b/src/main/scala/chisel3/util/experimental/BoringUtils.scala
index 36b3fb88..18551da8 100644
--- a/src/main/scala/chisel3/util/experimental/BoringUtils.scala
+++ b/src/main/scala/chisel3/util/experimental/BoringUtils.scala
@@ -1,4 +1,4 @@
-// See LICENSE for license details.
+// SPDX-License-Identifier: Apache-2.0
 
 package chisel3.util.experimental
 
diff --git a/src/main/scala/chisel3/util/experimental/Inline.scala b/src/main/scala/chisel3/util/experimental/Inline.scala
index 7e8a35fb..1d5fcb89 100644
--- a/src/main/scala/chisel3/util/experimental/Inline.scala
+++ b/src/main/scala/chisel3/util/experimental/Inline.scala
@@ -1,4 +1,4 @@
-// See LICENSE for license details.
+// SPDX-License-Identifier: Apache-2.0
 
 package chisel3.util.experimental
 
diff --git a/src/main/scala/chisel3/util/experimental/LoadMemoryTransform.scala b/src/main/scala/chisel3/util/experimental/LoadMemoryTransform.scala
index 4beafa19..d91d97b7 100644
--- a/src/main/scala/chisel3/util/experimental/LoadMemoryTransform.scala
+++ b/src/main/scala/chisel3/util/experimental/LoadMemoryTransform.scala
@@ -1,4 +1,4 @@
-// See LICENSE for license details.
+// SPDX-License-Identifier: Apache-2.0
 
 package chisel3.util.experimental
 
diff --git a/src/main/scala/chisel3/util/experimental/group.scala b/src/main/scala/chisel3/util/experimental/group.scala
index 87214ca9..e43115d0 100644
--- a/src/main/scala/chisel3/util/experimental/group.scala
+++ b/src/main/scala/chisel3/util/experimental/group.scala
@@ -1,4 +1,4 @@
-// See LICENSE for license details.
+// SPDX-License-Identifier: Apache-2.0
 
 package chisel3.util.experimental
 
diff --git a/src/main/scala/chisel3/util/random/FibonacciLFSR.scala b/src/main/scala/chisel3/util/random/FibonacciLFSR.scala
index c61f0210..32f5caa8 100644
--- a/src/main/scala/chisel3/util/random/FibonacciLFSR.scala
+++ b/src/main/scala/chisel3/util/random/FibonacciLFSR.scala
@@ -1,4 +1,4 @@
-// See LICENSE for license details.
+// SPDX-License-Identifier: Apache-2.0
 
 package chisel3.util.random
 
diff --git a/src/main/scala/chisel3/util/random/GaloisLFSR.scala b/src/main/scala/chisel3/util/random/GaloisLFSR.scala
index 85a6afde..0d407c87 100644
--- a/src/main/scala/chisel3/util/random/GaloisLFSR.scala
+++ b/src/main/scala/chisel3/util/random/GaloisLFSR.scala
@@ -1,4 +1,4 @@
-// See LICENSE for license details.
+// SPDX-License-Identifier: Apache-2.0
 
 package chisel3.util.random
 
diff --git a/src/main/scala/chisel3/util/random/LFSR.scala b/src/main/scala/chisel3/util/random/LFSR.scala
index 17e10311..08b124f6 100644
--- a/src/main/scala/chisel3/util/random/LFSR.scala
+++ b/src/main/scala/chisel3/util/random/LFSR.scala
@@ -1,4 +1,4 @@
-// See LICENSE for license details.
+// SPDX-License-Identifier: Apache-2.0
 
 package chisel3.util.random
 
diff --git a/src/main/scala/chisel3/util/random/PRNG.scala b/src/main/scala/chisel3/util/random/PRNG.scala
index 37f50599..d94b78e8 100644
--- a/src/main/scala/chisel3/util/random/PRNG.scala
+++ b/src/main/scala/chisel3/util/random/PRNG.scala
@@ -1,4 +1,4 @@
-// See LICENSE for license details.
+// SPDX-License-Identifier: Apache-2.0
 
 package chisel3.util.random
 
diff --git a/src/main/scala/chisel3/util/util.scala b/src/main/scala/chisel3/util/util.scala
index 164abdf8..6ea1f629 100644
--- a/src/main/scala/chisel3/util/util.scala
+++ b/src/main/scala/chisel3/util/util.scala
@@ -1,4 +1,4 @@
-// See LICENSE for license details.
+// SPDX-License-Identifier: Apache-2.0
 
 package chisel3