device-tree: Add recipe for creating device tree blobs
device-tree: Add recipe for creating device tree blobs

* Using the linux-dtb.inc steps, create a recipe external from the
  kernel recipe to handle the building of device tree blobs.
* For backwards compatibility create a symlink with the kernel imagetype

Signed-off-by: Nathan Rossi <e1e6d173c6873c0ae2b831215d22380d293bb314@xilinx.com>

nathanrossi/meta-xilinx,Xilinx/meta-xilinx,Xilinx/meta-xilinx,nathanrossi/meta-xilinx,nathanrossi/meta-xilinx,Xilinx/meta-xilinx,Xilinx/meta-xilinx,Xilinx/meta-xilinx,Xilinx/meta-xilinx,nathanrossi/meta-xilinx