armv8r64-extras/xen-u-boot-scr: Add recipe to generate U-Boot script

Add a standalone recipe to generate the U-Boot boot script for Xen
dom0less instead of the bbappend and bbclass. This recipe calls mkimage
directly, instead of relying on do_compile in the U-Boot recipe in
OE-core.

Make xen_dom0less_image.bbclass depend on the new recipe, instead of
U-Boot itself. This ensures that the firmware is not recompiled after
modifying the configuration of the Xen dom0less stack.

Issue-Id: SCM-4304
Signed-off-by: Peter Hoyes <Peter.Hoyes@arm.com>
Change-Id: I865d049a9ef514fcf0ccbf051fd98ab06de1805d