gluon-client-bridge: add LAN interfaces to client bridge by default (instead of doing this in the batman-adv-specific scripts)

This allows to weaken the gluon-mesh-batman-adv-core dependency of
gluon-luci-portconfig to gluon-client-bridge.

Some drivers (mt76) don't support arbitrary MAC addresses. Use the
addresses provided by the driver (avoiding the primary address) by default,
but fall back to our has-based scheme when the driver doesn't provide
(enough) addresses.