This will help to move non-secure and secure boot closer. It will also
be required when booting non-x86 kernels with device trees that shall be
replaceable via updates, thus shall be embedded into the unified kernel
image.

The change obsoletes the need to specify "unified-kernel=y" as
efibootguard-boot plugin parameter. Users can still select the classic
boot method by providing "unified-kernel=n".

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