wic: add UEFI kernel as UEFI stub

Linux kernel can be compiled as UEFI stub and loaded directly
with UEFI firmware without grub or other UEFI shell.

Tested with wic file:
bootloader  --ptable gpt --timeout=0  --append="rootwait"
part /boot --source bootimg-efi --sourceparams="loader=uefi-kernel" \
 --ondisk sda  --fstype=vfat --label bootfs \
  --active --align 1024 --use-uuid
part / --source rootfs --fstype=ext4 --label rootfs \
  --align 1024 --exclude-path boot/ --use-label

Signed-off-by: Maxim Uvarov <maxim.uvarov@linaro.org>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>