[TF-M] BL2 builds, no "PYTHON_EXECUTABLE" defined

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