ramips: enable BR6425N in kernel config

Though originally contained in the patch applied in r35020, the move to 3.7 in
r34845 lost the kernel .config option. Reintroduce it to config-3.7.

Signed-off-by: Daniel Golle <dgolle@allnet.de>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@35191 3c298f89-4303-0410-b956-a3cf2f4a3e73