RFE: Flag to tell kiwi to create UEFI-only images for x86_64

[Conan-Kudo]

Problem description

As part of the effort to support UKIs in Fedora Cloud, we need to be able to create images that are not hybrid UEFI+BIOS, but instead are UEFI-only. Currently, when setting an image firmware to efi or uefi, we produce hybrid images and image builds fail if we do not provide GRUB2 for BIOS boot.

One way to solve this would be to add a uefi_nocsm variant to not produce hybrid boot images would resolve this issue. Another way would be to add a csm= flag that defaults to on but when set to off would not produce hybrid images.

OS and Software information

• KIWI version: 9.25.16
• Operating system host version: Fedora Linux 40
• Operating system target version: Fedora Linux 40
• Open Build Service version (N/A if not using OBS): N/A
• Koji version (N/A if not using Koji): 1.33.1

[Conan-Kudo]

cc: @davdunc @kraxel

[Conan-Kudo]

cc: @davide125 @DaanDeMeyer

[Conan-Kudo]

I'm personally leaning toward adding a csm= flag rather than another weird uefi mode, especially since we want to simplify the uefi firmware modes for kiwi 10.

[kraxel]

grub is no problem, it is not used anyway ...
<bootloader name="custom"/>
editbootconfig = https://pagure.io/fork/kraxel/fedora-kiwi-descriptions/blob/kraxel/f/uki-setup.sh

[Conan-Kudo]

Having csm=off would also allow us to disable creating the BIOS boot partition when it's unneeded.