batman-adv: make GW election code protocol specific

Each routing protocol may have its own specific logic about
gateway election which is potentially based on the metric being
used.

Create two GW specific API functions and move the current election
logic in the B.A.T.M.A.N. IV specific code.

Signed-off-by: Antonio Quartulli <a@unstable.cc>
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Marek Lindner <mareklindner@neomailbox.ch>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>