apm821xx: make it possible to update the dtb partition on the WNDR4700

apm821xx: make it possible to update the dtb partition on the WNDR4700

Currently, the device-tree partition is marked as read-only.
Hence, userspace tools like mtd can't write into the partition.
This however will be necessary in case the DTB needs to be
updated.

This patch also adds the kernel.dtb image, so the compiled
DTB is exported as a file and available in the binary
directory along the firmware images.

Note: the u-boot does expects the dtb to be a uimage.

To update the dtb manually:
 1. copy the generated dtb to the router.
 2. mtd erase /dev/mtd2
 3. mtd write wndr4700.dtb /dev/mtd2

Signed-off-by: Christian Lamparter <chunkeey@gmail.com>