ar71xx: image: update Image/Build/WHRHPG300N macro

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

Tested on the WHR-G301N board only.

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

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@38141 3c298f89-4303-0410-b956-a3cf2f4a3e73