uki.bbclass: make DTB packaging optional

uki.bbclass: make DTB packaging optional

According to the Unified Kernel Image (UKI) specification [1], including
a compiled device tree in the .dtb section of a UKI is optional. However,
the current implementation always packages device trees listed in
KERNEL_DEVICETREE into the UKI by default. This makes uki.bbclass
unsuitable for systems that rely on firmware-provided DTBs and do not
want a DTB embedded in the UKI.

Fix this by introducing a new UKI_DEVICETREE variable to control device
tree packaging. The .dtb section is now populated from UKI_DEVICETREE
instead of KERNEL_DEVICETREE. Users who do not want DTBs included in the
UKI can override UKI_DEVICETREE to an empty value from their recipes.

Update the UKI selftests accordingly, as QEMU does not provide a device
tree to embed.

[1] https://uapi-group.org/specifications/specs/unified_kernel_image/

Signed-off-by: Viswanath Kraleti <viswanath.kraleti@oss.qualcomm.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>