nixos/systemd-boot: switch to UKI from bzImage

[Prince213]

systemd-boot still uses bzImage. It would be nice if we could use UKIs instead, which would be helpful for secure boot.

nixpkgs/nixos/modules/system/boot/loader/systemd-boot/systemd-boot-builder.py

Line 165 in f9969bf

kernel = copy_from_file(bootspec.kernel)

Related: #413039

[Prince213]

For the current appoach to discover UKIs, we need this information in Bootspec (NixOS/rfcs#165).

[Prince213]

We are currently using the "kernel" specified in Bootspec v1 (here), which is a bzImage.

nixpkgs/nixos/modules/system/boot/loader/systemd-boot/systemd-boot-builder.py

Lines 196 to 202 in 6425d59

	f.write(BOOT_ENTRY.format(title=title,
	sort_key=bootspec.sortKey,
	generation=generation,
	kernel=kernel,
	initrd=initrd,
	kernel_params=kernel_params,
	description=f"{bootspec.label}, built on {build_date}"))