* Move all scripts and documentation to Python3
Python 2 EOL is Jan 1, 2020. Migrate scripts to run under Python 3.
Under Windows, we're already running Python 3.7, by dumb luck. The
oddness is that the Windows standalone executable for Python 3 is called
"python" whereas under UNIX-like OSes it's called "python3" with
"python" always referring to the Python 2 executable. The ZIP needs to
be updated to include a Python3.exe (copy of Python.exe) so that we can
use the same command lines under Linux and Windows, and to preserve my
sanity.
Fixes#6376
* Add new Windows ZIP with python3.exe file
* Sort options in boards.txt generation for repeatability
The order of the board opts dict changes depending on the Python version
and machine, so sort the options before printing them to get a stable
ordering.
* Re-add Python2 compatibility tweaks
Most scripts can run as Python 2 or Python 3 with minimal changes, so
re-add (and fix, as necessary) compatibility tweaks to the scripts.
This allows requesting environment variable values set in the C++ test
code, from python mock code. Use case is cross-checking test
results against values expected by the host side part of the test.
* tests/device/Makefile: use bourne shell compatible "." instead ot "source" (ubuntu's dash does not understand it)
* tests/device: make BEGINTIMEOUT a new error return value instead of a test-process-breaking exception
