gluon-mesh-batman-adv-core: make mesh_lan config a bridge

There are a few devices which have more than one LAN interface (for example
some revision of the TL-WR941ND, which uses a DSA-based switch, so each
switch port has its own netdev.) On these devices we need a bridge for
mesh-on-lan (as the alternative of adding them to batman-adv individually
would need too many MAC addresses.)