wic/bootimg-efi: Factor out some common bits

wic/bootimg-efi: Factor out some common bits

The paths for configuring grub and systemd-boot have some common bits
around copying the initrd files. This will even grow when adding dtb
support. Factor this out into a class function.

Along this, avoid evaluating 'create-unified-kernel-image' multiple
times in do_configure_systemdboot and suppress a bogus warning about
"Ignoring missing initrd" when it is turned on.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>