kernel-fitimage.bbclass: Introduce a way to provide external dtb
Currently kernel-fitimage gets the dtb from KERNEL_DEVICETREE, however
there are instances when the DTB file can be generated using other
recipes, provide a way to include the DTB instead of inkernel device
tree.
Add support for external device tree during the creation of fitimage. If
virtual/dtb is set using PREFERRED_PROVIDER_virtual/dtb = "devicetree", which
inherits the devicetree.bbclass then use the path provided in
EXTERNAL_KERNEL_DEVICETREE else use KERNEL_DEVICETREE during fitimage process
Signed-off-by: Manjukumar Matha <manjukumar.harthikote-matha@xilinx.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
