scripts: kconfig: Add copyright and license to kconfig.py

The file kconfig.py was originally introduced by myself in:
https://github.com/zephyrproject-rtos/zephyr/pull/5569



The file was a modified version of a Kconfiglib one, so it makes sense
to keep the original license (ISC).

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