ar71xx: image: use a common macro for Buffalo WZRHP devices

The Image/Build/WZRHP64K and the Image/Build/WZRHP128K macros
are similar. Move the common stuff into the Image/Build/WZRHP
macro and update the orignal macros to use that.

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

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@38152 3c298f89-4303-0410-b956-a3cf2f4a3e73
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>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@38153 3c298f89-4303-0410-b956-a3cf2f4a3e73