ar71xx: update the Image/Build/WZRHP macro

 - append the root squashfs directly after the kernel,
 - use the 64kraw/128kraw template instead of the 64k/128k,
 - ensure that the generated images have enough erase blocks
   for JFFS2 garbage collection,

Tested on WZR-HP-G300NH and WZR-600DHP boards.

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

SVN-Revision: 38153