this means that net devices in devices/foo/default.nix can be
specified by their sysfs paths (instead of by "eth0" and "eth1" that
may change from one kernel version to the next) and given mnenomic
names that are helpful for the hardware. Like "wan" and "lan[1..4]"
... and make bridge use it.
We also had to convert bridge back into a pair of services.
Downstreams want to depend on the bridge it self being configured
even if not necessarily all the members are up. e.g. don't want
to break ssh on lan if there's a misconfigured wlan device
