batman-adv: Add bonding mesh genl configuration

The mesh interface can use multiple slave/hard-interface ports at the same
time to transport the traffic to other nodes.

The BATADV_CMD_SET_MESH/BATADV_CMD_GET_MESH commands allow to set/get the
configuration of this feature using the BATADV_ATTR_BONDING_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>