While attempting to build for the musca_s1, with mcuboot, on
    Windows, the build failed very late and brought up a Code Writer
    window on my screen.
    
    I nailed it down to no "PYTHON_EXECUTABLE" being defined while
    processing bl2/ext/mcuboot/CMakeLists.txt, which caused the build
    line to just attempt to run wrapper.py, with no interpreter, and
    windows brought up the appropriate tools to write python code.
    
    Adding "-DPYTHON_EXECUTABLE=python" to the cmake line fixed that,
    but it seems that this shouldn't be needed. The build tools should
    handle this, or?
    
    There is a line:
    #! /usr/bin/env python3
    
    in wrapper.py, and I assume linux will handle this appropriately,
    but it doesn't seem windows does.
    
    Cheers,
    Thomas