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.

Avoid commiting configs inbetween to decrease number of flash writes.

Fixes #714.

This package provides br-client and sets up a wireless AP interface for
clients.