add bridge priority option

add bridge priority option

[netifd] add bridge priority option

Using the bridge priority (lower numbers are higher in the
hierarchy), one can ensure that the router is chosen as root
bridge in a setup with spanning tree protocol.

For instance, one can set the priority of network lan to 32767,
causing the router to win over all directly and indirectly
connected nodes that have a default priority of 32768.

The reason for doing that is that otherwise it has a default
priority 32768 as well and any other connected node could win and
get root bridge. In a home setup, those nodes are often desktop
or laptop boxes and get switched off from time to time. As a
consequence, root bridges vanish or new root bridges get chosen
relatively often, resulting in frequent topology changes to the
STP network. While the new topology has not settled, packets can
get lost, causing noticeable interruptions of network traffic.

Setting the router's bridge on a lower numbered priority (and thus
higher in the selection hierarchy) solves the problem in the vast
majority of the cases by ensuring that the device that is most
likely powered on 24/7 gets chosen as root and prevents topology
changes.

Signed-off-by: Niels Boehm <blubberdiblub@gmail.com>