scripts: compliance.py updated to support KconfigBasic

This commit lets the Kconfig compliance check run from top of current
git repo instead of only executing inside ZEPHYR_BASE.
This extends the usability of the Kconfig compliance check.

Together with this possibility, a KconfigBasic mode has been added which
will execute the same verification for symbols in the Kconfig tree but
will not check for undefined symbols outside Kconfig.

This is needed as running Kconfig undef check outside the tree in
non-Zephyr repos might pickup `CONFIG_` named symbols in other code.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>