ar71xx: image: update Build/Image/Planex macro

ar71xx: image: update Build/Image/Planex macro

 - remove the 'kernel' and 'rootfs' partitions from the mtd layout,
 - move the .okli image into the uImage,
 - 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 MZK-W300NH board only.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@38139 3c298f89-4303-0410-b956-a3cf2f4a3e73