gluon-mesh-batman-adv-core: Always forward towards mesh/batman-adv

An IGMP/MLD domain split will prevent us from being able to track
multicast listeners on other nodes.

Therefore we need to always hand any multicast packets we received from
local clients to batman-adv. With bridge multicast snooping disabled,
the current setting in Gluon, this is already the case.

However, in preparation to enabling multicast snooping, we need to
enforce forwarding towards batman-adv by setting the bridge port
option "multicast_router" to 2.

Signed-off-by: Linus Lüssing <linus.luessing@c0d3.blue>