This is to keep the top-level directory clean, to make it clear what is
core and what is a port, and to allow the repository to grow with new ports
in a sustainable way.

$(TOP) is defined in py/mkenv.mk and should be used to refer to the top
level of this repository.

It provides sys.stdin, sys.stdout, sys.stderr for bare-metal targets based
on mp_hal functions.

This follows the pattern of how all other headers are now included, and
makes it explicit where the header file comes from.  This patch also
removes -I options from Makefile's that specify the mp-readline/timeutils/
netutils directories, which are no longer needed.

The -ansi flag is used for C dialect selection and it is equivalent to -std=c90.
Because it goes right before -std=gnu99 it is ignored as for conflicting flags
GCC always uses the last one.

- add template rule that converts a specified source file into a qstring file

- add special rule for generating a central header that contains all
  extracted/autogenerated strings - defined by QSTR_DEFS_COLLECTED
  variable. Each platform appends a list of sources that may contain
  qstrings into a new build variable: SRC_QSTR. Any autogenerated
  prerequisities are should be appened to SRC_QSTR_AUTO_DEPS variable.

- remove most qstrings from py/qstrdefs, keep only qstrings that
  contain special characters - these cannot be easily detected in the
  sources without additional annotations

- remove most manual qstrdefs, use qstrdef autogen for: py, cc3200,
  stmhal, teensy, unix, windows, pic16bit:

   - remove all micropython generic qstrdefs except for the special strings that contain special characters (e.g. /,+,<,> etc.)
   - remove all port specific qstrdefs except for special strings
   - append sources for qstr generation in platform makefiles (SRC_QSTR)

Scenario: module1 depends on some common file from lib/, so specifies it
in its SRC_MOD, and the same situation with module2, then common file
from lib/ eventually ends up listed twice in $(OBJ), which leads to link
errors.

Make is equipped to deal with such situation easily, quoting the manual:
"The value of $^ omits duplicate prerequisites, while $+ retains them and
preserves their order." So, just use $^ consistently in all link targets.

Generally, ports should inherit INC from py.mk, append to it, not
overwrite it. TODO: Likely should do the same for other vars too.