ipconfig: Handle devices which take some time to come up.

Some network devices, particularly USB ones, take several seconds to
fully init and appear in the device list.

If the user turned ipconfig on, they are using it for NFS root or some
other early booting purpose.  So it makes no sense to just flat out
fail immediately if the device isn't found.

It also doesn't make sense to just jack up the initial wait to
something crazy like 10 seconds.

Instead, poll immediately, and then periodically once a second,
waiting for a usable device to appear.  Fail after 12 seconds.

Signed-off-by: David S. Miller <davem@davemloft.net>
Tested-by: Christian Pellegrin <chripell@fsfe.org>