net: ieee802154: Add common config for all 802154 L2s

Introduce a common config for all 802.15.4 based L2 implementations.
This way, any custom 15.4 L2 implementation will be able to
automatically enable use 15.4 driver, w/o a need to modify the actual
Kconfig.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>