This speeds up the build process in many cases because this file is
refreshed on every build and by making it a code file much less off the
build graph depends on it.
Windows 10 includes since version 1903 a python stub which is supposed
to open the MS Store if python is invoked but not installed. However
when invoked from tup, the stub simply hangs without output.
With this change we first check the python version (something that the
Windows stub does not interer with) before we invoke it.
build requirements (like multiple active board versions).
Developers will need to install tup, as described in the README.
A stub Makefile is still available for convenience, so the main
"make ..." commands can still be used.