ieee802154: handle config as menuconfig

This patch handles the IEEE802154 Kconfig entry as menuconfig.
Furthermore we move this entry out of "Network Options" and put it into
"Networking" where the other networking subsystems are. This requires a
menuconfig entry like all other networking subsystems.

Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>