batman-adv: Add bridge_loop_avoidance mesh genl configuration

The mesh interface can try to detect loops in the same mesh caused by
(indirectly) bridged mesh/soft-interfaces of different nodes. Some of the
loops can also be resolved without breaking the mesh.

The BATADV_CMD_SET_MESH/BATADV_CMD_GET_MESH commands allow to set/get the
configuration of this feature using the
BATADV_ATTR_BRIDGE_LOOP_AVOIDANCE_ENABLED attribute. Setting the u8 to zero
will disable this feature and setting it to something else is enabling this
feature.

Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>