bridge: make learning and unicast-flood configurable per bridge port

bridge: make learning and unicast-flood configurable per bridge port

Tuning these two options allows a more fine grained configuration of the
forwarding database (fdb) of a bridge.

The former allows to enable or disable the learning of the presence of
MAC addresses behind a bridge port. (default: enabled on all ports)

The latter allows to tune the behaviour in case a destination MAC address
of a frame is unknown to the fdb, like only flooding on specific ports or
not flooding on any port. (default: flood on all ports, except incoming)

This can be useful to create a dumb hub, for instance for monitoring
purposes. Or in larger layer 2 mesh networks to avoid keeping redundant
databases (e.g. with the batman-adv translation table).

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