This allows multiple versions (e.g. Debug/Release, x86/x64) of micropython.exe
to co-exist instead and also solves potential problems where msbuild does not
completely rebuild the output and/or pdb files when switching between builds,
which in turn can cause linker errors in dependent projects.
By default exe/map/... files go in windows/build/$(Configuration)$(Platform)
After each build micropython.exe is still copied from the above directory to
the windows directory though, as that is consistent with the other ports and
the test runner by default uses that location as well.
Also rename env.props -> path.props which is a clearer name,
and add ample documentation in the affected build files.
(also see discussion in #1538)
stijn