Extend the script by some code from the official list_undefined.py example[1]
to further detect symbols we are using, but haven't defined anywhere.

[1]: https://github.com/ulfalizer/Kconfiglib/blob/35a60b7/examples/list_undefined.py

Signed-off-by: Ahmad Fatoum <ahmad@a3f.at>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>