igep003x: Add netboot support

netboot allows to boot an external image using TFTP and NFS protocols

Signed-off-by: Pau Pajuelo <ppajuelo@iseebcn.com>
Signed-off-by: Ladislav Michl <ladis@linux-mips.org>
Tested-by: Pau Pajuelo <ppajuel@gmail.com>