scripts: kconfig-lint.py: extend for undefined symbol detection ...

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>

scripts: import Zephyr scripts/kconfig/lint.py into barebox ...

This kconfig linter can do some useful analysis to find problems
in our Kconfig files. Import it into barebox source tree with the
changes necessary to be usable.

The results of running it should be taken with a grain of salt and
verified with grep.

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