ar71xx: simplify wndr3700 image generation

 - remove the kernel and rootfs partitions from the mtd layout,
   the kernel creates those automatically,
 - remove estimation step, it is not needed anymore
 - put the root squashfs directly after the kernel
 - use the 64kraw template

With the default configuration, this allows to use more space for
rootfs_data with 64KiB. Tested on WNDR3700 v1 only.

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

SVN-Revision: 38120