init_systap: change lookup method for bridge
init_systap: change lookup method for bridge

change lookup method from hard-coded "bridgeXX" iface to searching by description and
bridge-type first. it makes possible to use bridge with custom non-"bridge" iface name
while Im here, add a hint for what init_systap() is needed.

@mekanix can you review this please?

olevole/cbsd,olevole/cbsd,cbsd/cbsd,cbsd/cbsd,olevole/cbsd,cbsd/cbsd,cbsd/cbsd,olevole/cbsd