ar71xx: image: update the Image/Build/CameoHornet macro

 - append the root squashfs directly to the kernel,
 - use the 64kraw-nojffs template instead of sqashfs-only
 - ensure that the generated images have enough erase blocks
   for JFFS2 garbage collection,

Tested on a TEW-712BR devices only.

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

SVN-Revision: 38130