In barebox network packets always go out at the current ethernet
device and are expected to be received from the current interface.
This has some side effects. When for example an NFS is mounted when
one interface is active and the interface is changed afterwards the
NFS packets leave the new interface, but the NFS server won't be
reachable there.

Instead of changing the whole network traffic to the current ethernet
interface we now initialize a network connection with the current
network interface, but then the connection will continue to use that
interface even when the current interface is changed.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>