ar71xx: update the Image/Build/Cameo macro

 - append the root squashfs directly to the kernel,
 - use the 64kraw/64kraw-nojffs template instead of 64k/squashfs-only,
 - ensure, that the signature is placed at the correct offset in
   the *-factory images,
 - ensure that the generated images have enough erase blocks
   for JFFS2 garbage collection,

Tested on DIR-600-A1 and TEW-632BR devices only.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@38129 3c298f89-4303-0410-b956-a3cf2f4a3e73