bridge: Allow setting multicast_to_unicast option

bridge: Allow setting multicast_to_unicast option

With this patch the multicast_to_unicast feature can be disabled for all
wireless interfaces via an according option on the uci bridge interface.

This patch also exports the setting information to wireless handler
scripts. The hostapd script will need that information to determine
whether to enable or disable ap-isolation, for instance.

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