kconfig: Rebase to latest Kconfiglib

A lot of improvements have made it into Kconfiglib since we introduced
it into Zephyr. This rebases our 2 Zephyr-specific commits into the
latest Kconfiglib upstream.

Origin: https://github.com/zephyrproject-rtos/Kconfiglib/tree/zephyr



Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>