cmake: convert WEST_PYTHON path to be correct CMake style

The `WEST_PYTHON` reported by west (python) may use a different path
style (casing and separator) than the path style in CMake
find_package(Python3).

This may result in following additional error text when west version is
not meeting the minimum required version:

    The Python version used by west is:  c:\python37\python.exe
    The Python version used by CMake is: C:/Python37/python.exe
    This might be correct, but please verify your installation.

Using `get_filename_component` will return the path in same style as
CMake find_package(Python3).

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>