[NW scripts] Choose alternative interface if NFSroot is used.

[NW scripts] Choose alternative interface if NFSroot is used.

Allow the network bridging setup script to choose an alternative
network interface if it is determined that the root is mounted using
NFS, which is assumed to be done over the NIC serving the default
gw. In this case the prefix of the device is used to find other
devices, i.e., if eth0 is the default device an alternative device may
be an other ones starting with 'eth'.

Signed-off-by: Stefan Berger <stefanb@us.ibm.com>