cmake: Fix python version detection

The 'FindPythonInterp' that is distributed with CMake 3.8 is affected
by bug: https://github.com/zephyrproject-rtos/zephyr/issues/11103



To work around this, until we upgrade to 3.13, we copy and patch the
3.8 version of 'FindPythonInterp' into the Zephyr repository.

Signed-off-by: Sebastian Bøe <sebastian.boe@nordicsemi.no>