Switch to the unified kernel image and its generator script that EFI
Boot Guard now provides. So far this only simplifies the generation
process. But it will also allow to use the more advanced device tree
embedding of the EBG linux-stub later on.

As the linux-stub uses LINUX_EFI_INITRD_MEDIA_GUID for loading the
initrd, we need to restrict support to CIP kernel 5.10 as this feature
was only introduced with 5.8.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>