This makes it easier to distribute prefixes over a wireguard tunnel
interface, by simply setting the ip6prefix option in uci (just like with
other protocols).
Obviously, routing etc needs to be setup properly for things to work; this
just adds the config option so the prefix can be assigned to other
interfaces.
Signed-off-by: Toke Høiland-Jørgensen <toke@toke.dk>