libbatadv: Add library for common batman-adv helpers

Interacting with batman-adv's genl interface requires some code and
definitions which could be shared between different packages. libbatadv is
trying to do this without providing any guarantee for ABI or API stability.
It is only useful in very controlled environments like gluon.

Signed-off-by: Sven Eckelmann <sven@narfation.org>