lantiq: use upstream mechanism to append device tree file

Instead of using our patch-dtb program just place the device tree
behind the kernel binary and then let the in kernel mechanism fetch it.
This also adds support for having the device tree file in the boot
loader.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@49050 3c298f89-4303-0410-b956-a3cf2f4a3e73