wic/bootimg-efi: Add support for loading devicetree files

wic/bootimg-efi: Add support for loading devicetree files

For device tree using systems, add support to set a custom devices tree
during UEFI boot. This requires to copy the DTB file to the boot
partition and to add the respective loader entries to the configuration
files.

Both grub and systemd-boot support only loading a specific device tree.
Therefore refuse to work if the 'dtb' parameter contains more than one
entry.

Out of scope for now are overlays (only supported by systemd-boot).

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
[Luca: rebased on commit 98e9d435b278 ("wic/bootimg-efi: use cross objcopy when building unified kernel image")]