ar71xx: create image for the WNDR3800

This adds support for the Netgear WNDR3800, it is almost the same as
the WNDR3700v2. It just has more RAM so the hd_id had to be adjusted.

Signed-off-by: Petri Rosenström <petri.rosenstrom@gmail.com>

SVN-Revision: 28597