gluon-mesh-batman-adv: announce MAC of mesh ifaces

This will make a node announce all MACs of its interfaces participating
in the batman-adv mesh. This enables other nodes to associate the
announced object with both the data reported by batadv-vis as well as a
simple list of neighbours as output by `iw dev $IFACE station dump`.
Merge branch 'mesh-announce-interfaces' of https://github.com/freifunk-gluon/packages