kernel-fitimage.bbclass: support both KERNEL_DEVICETREE/EXTERNAL_KERNEL_DEVICETREE

There are user cases that a developer wants to add both in-tree and
out-of-tree devicetrees to fitimage, and this is quite normal, for
instance, a developer might need the devicetrees from kernel source
meanwhile maintaining his/her own devicetree overlays to be applied
on top of them.

To support that, we now allow KERNEL_DEVICETREE and
EXTERNAL_KERNEL_DEVICETREE to be both set in configuration files, and
the devicetrees in EXTERNAL_KERNEL_DEVICETREE have higher priority to
override the same names in KERNEL_DEVICETREE.

Signed-off-by: Ming Liu <liu.ming50@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>