This renames batman-adv.ko to batman-adv-legacy.ko. And also moves
scripts to a "-legacy" variant to avoid build errors when both
batman-adv and batman-adv-legacy are selected. Finally, moves the
batman-adv-legacy config to /etc/config/batman-adv-legacy.
This allows to select and install the batman-adv mainline and
batman-adv-legacy simultaneously.