"A unified kernel image is a single EFI PE executable combining an EFI
stub loader, a kernel image, an initramfs image, and the kernel command
line.
[...]
Images of this type have the advantage that all metadata and payload
that makes up the boot entry is monopolized in a single PE file that can
be signed cryptographically as one for the purpose of EFI
SecureBoot."[1]
This commit adds a create-unified-kernel-image=true option to the
bootimg-efi plugin for creating a Unified Kernel Image[1] and installing
it into $BOOT/EFI/Linux/ with a .efi extension per the the Boot Loader
Specification[1][2]. This is useful for implementing Secure Boot.
systemd-boot is the only mainstream bootloader implementing the
specification, but GRUB should be able to boot the EFI binary, this
commit however doesn't implement the necessary changes to the GRUB
config generation logic to boot the Unified Kernel Image.
