armv8r64-extras/u-boot: Apply Xen overlay device tree at boot

To avoid modifying the firmware's device tree with the Xen additions,
the generated device tree overlay will be stored in the boot partition,
from which it will be loaded and applied by the U-Boot boot script.

Modify xen_dom0less_image.bbclass to inherit xen_dom0less_config, and
add the overlay device tree to the boot partition so it can be picked up
by U-Boot.

Modify the U-Boot boot script to load and apply the overlay at boot.

Issue-Id: SCM-4386
Signed-off-by: Peter Hoyes <Peter.Hoyes@arm.com>
Change-Id: Idcc5a0cf4dbdc4e4fc967a77d19bc88dff1045c9