[PATCH v2] Python2/Python3 support

[chrisj at rtems.org]

The patch implements runtime support to select a python to run the RSB.
The selection order is:

  python2
  python3
  python

In a future release we can change this order without needing to change
the support structures around the RSB.

The patch also handles GDB's selection of python moving away from the
automatic selection in GDB to forcing a selection. The patch allows
a host to provide a selection, for example Windows where we need 
GCC built python development libraries, a build of GDB forcing a version
to build with or the GDB common script selecting a version of Python. 

The python wrapper has testing support in the form of an environment
variable to override the automatic selection to force a version of python:

 export RTEMS_PYTHON_OVERRIDE=python3 

I have tested a build on FreeBSD, MSYS2 and MacOS. I have tested python2
and python3 on MSYS2 using an MSYS built Python.

Please test this patch on your OS, specifically Linux and let me know.
I would like to push this change as soon as possible.

I suggest you use the `--dry-run` option as an initial test. The patch
attempts to find the Python.h header file and the python library. This
part of the change may be a little fragile to start with until I get 
feedback from a number of hosts.

Chris