batman-adv: B.A.T.M.A.N. V - implement bat_neigh_print API

Lists all neighbours detected by the Echo Locating Protocol
(ELP) and their throughput metric.

Initially Developed by Linus during a 6 months trainee study
period in Ascom (Switzerland) AG.

Signed-off-by: Linus Luessing <linus.luessing@web.de>
Signed-off-by: Marek Lindner <mareklindner@neomailbox.ch>
Signed-off-by: Antonio Quartulli <antonio@open-mesh.com>