configure_networking(): Wait for specified network device

configure_networking() will issue a udevadm settle before trying to configure an interface. However it is possible for a NIC to appear after the event queue has emptied. This is pretty reproducible with a USB NIC I have. There doesn't appear to be a good way to wait for all devices that were connected at boot time to become available - udevadm settle is just the closest thing we have.

However, in the case that the user has told us which interface they expect to be used in the initramfs (via ip=), we can just wait for it specifically. As a bonus, this should shave off some time on systems where the NIC shows up before the udev event queue has emptied, as we can avoid the udevadm settle.

Closes: #965935 Signed-off-by: dann frazier dannf@debian.org