ppc4xx: Update configs for Netstal boards

I reorganized my config files, putting the common stuff into netstal-common.h
(got the idea by looking a amcc-common.h from Stefan).

Added stuff to boot the new powerpc linux via NFS (only tested with HCU4).

Signed-off-by: Niklaus Giger <niklaus.giger@netstal.com>
Signed-off-by: Stefan Roese <sr@denx.de>