Define the SILK board dependent part of the Ether device node.
Enable DHCP and NFS root for the kernel booting.

Based on the original patch by Vladimir Barinov
<vladimir.barinov@cogentembedded.com>.
Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>