wic bootimg-efi.py: change UKI support from wic plugin to uki.bbclass

wic bootimg-efi.py: change UKI support from wic plugin to uki.bbclass

Remove custom wic plugin implementation and use systemd ukify reference
implementation when generating UKI images. Fail if users still have
create-unified-kernel-image in wic image config. uki.bbclass use is
detected from IMAGE_CLASSES variable ("inherit uki" in image
recipe) so export that to wic plugins.

If UKI is used, then only generate a minimal loader config for
systemd-boot which basically just sets a timeout. Also set 5 second
timeout by default instead of failing if wic bootloader config is
missing. Boot menu is generated at runtime based on UKI binaries
found from ESP partition.

Signed-off-by: Mikko Rapeli <mikko.rapeli@linaro.org>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>