kconfig: Refactor kconfig.py to use __main__ and argparse

Kconfig.py is not following the de-facto (real?) coding standards of
Zephyr. This commit refactors kconfig.py with two changes:

Use __main__ and def main().

Use argparse instead of sys.argv.

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