kconfiglib: Update to fd21c5cb320b9

Update kconfiglib to revision fd21c5cb320b9 which can be found at
https://github.com/SebastianBoe/Kconfiglib

This allows us to eventually resolve some outstanding Kconfig issues
such as https://github.com/zephyrproject-rtos/zephyr/issues/4951

Signed-off-by: Sebastian Bøe <sebastian.boe@nordicsemi.no>
1 file changed