This is a way bigger/dumber bug than just bridge interfaces.
ifup is trying to bring up interfaces when udev hasn't even finished creating devices; before drivers have even loaded.
This is probably one of ifup's many other bugs, though I'm not sure which one. I'm still looking into this.
Suspect use of "udevadm settle" is inappropriate, though this could also be udevadm returning inappropriately soon. I'm not sure yet.
