scripts: ci: check_compliance: Account for board & SoC extensions

When a given board or SoC is extended, it has more than one directory
associated with it, where the build system can look for Kconfig files.
Make sure all of them are also included for Kconfig compliance checks.

Signed-off-by: Grzegorz Swiderski <grzegorz.swiderski@nordicsemi.no>