Add support for storing args during an exception raised by an irq.

The user code should call micropython.alloc_emergency_exception_buf(size)
where size is the size of the buffer used to print the argument
passed to the exception.

With the test code from #732, and a call to
micropython.alloc_emergenncy_exception_buf(100) the following error is
now printed:
```python
>>> import heartbeat_irq
Uncaught exception in Timer(4) interrupt handler
Traceback (most recent call last):
  File "0://heartbeat_irq.py", line 14, in heartbeat_cb
NameError: name 'led' is not defined
```

1 files changed, 9 insertions, 0 deletions

diff --git a/stmhal/mpconfigport.h b/stmhal/mpconfigport.h
index 95f142ca4..cbdfcf4d0 100644
--- a/stmhal/mpconfigport.h
+++ b/stmhal/mpconfigport.h
@@ -52,6 +52,15 @@
 #define MICROPY_PY_IO               (1)
 #define MICROPY_PY_IO_FILEIO        (1)
 
+#define MICROPY_ENABLE_EMERGENCY_EXCEPTION_BUF   (1)
+#define MICROPY_EMERGENCY_EXCEPTION_BUF_SIZE  (0)
+
+void enable_irq(void);
+void disable_irq(void);
+
+#define MICROPY_BEGIN_ATOMIC_SECTION()  disable_irq()
+#define MICROPY_END_ATOMIC_SECTION()    enable_irq()
+
 // extra built in names to add to the global namespace
 extern const struct _mp_obj_fun_native_t mp_builtin_help_obj;
 extern const struct _mp_obj_fun_native_t mp_builtin_input_obj;