cmake: Set PYTHON_EXECUTABLE env var with kconfig

In order to be able to invoke Python from Kconfig files,
set the path to the Python executable (which can be python,
python3, py -3, etc) as an environment variable so that
Kconfig shell invocations can use it like:

config MY_OPTION
       string "My option string"
       default "$(shell,$(PYTHON_EXECUTABLE) \
	          $(ZEPHYR_BASE)\scripts\script.py)"

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>