scripts: import Linux Kconfig.include ...

With this include, we can have goodies like $(cc-option ...) and
$(success ...) inside our Kconfig files.

gcc-version also now becomes available. While unused for now, it can
allow us to selectively turn on warnings on new GCC versions, which
were previously disabled because of their too high false positive rate.

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>