Support `bootupctl backend generate-metadata --format=2`

[HuijingHei]

See discussion in #995 (comment) and #995 (comment)

[travier]

$ podman run --rm -ti quay.io/fedora/fedora-coreos:43.20251214.2.0 rpm -qa | grep bootupd
bootupd-0.2.31-1.fc43.x86_64

$ podman run --rm -ti quay.io/fedora/fedora-coreos:43.20251214.2.0 ls /usr/lib/bootupd/updates /usr/lib/efi
ls: cannot access '/usr/lib/efi': No such file or directory
/usr/lib/bootupd/updates:
BIOS.json  EFI  EFI.json

$ podman run --rm -ti quay.io/fedora/fedora-coreos:43.20260105.2.0 rpm -qa | grep bootupd
bootupd-0.2.32-2.fc43.x86_64

$ podman run --rm -ti quay.io/fedora/fedora-coreos:43.20260105.2.0 ls /usr/lib/bootupd/updates /usr/lib/efi
/usr/lib/bootupd/updates:
BIOS.json  EFI.json

/usr/lib/efi:
grub2  shim

[travier]

So the consequence of step 2 from #926, implemented in #995 is that we moved the place where we store the content meant for the ESP and that caused regressions for various tools (bootc-image-builder) that look there to get them from the container image.

[travier]

We can decide to move things, but we should not move it during the lifecycle of a Fedora or CentOS release. Right now we have https://fedoraproject.org/wiki/Changes/BootLoaderUpdatesPhase1 planned for F44 so this should not change in F43 (and C10S, and below).

[cgwalters]

and that caused regressions for various tools (bootc-image-builder) that look there to get them from the container image.

Right that's e.g. osbuild/bootc-image-builder#1173

But I dunno...I think part of the problem is for some image build types they're basically bypassing bootupd, but we could think about how to actually use it instead there.

At a minimum it'd likely be simple for us to add the EFI vendor as a metadata property.

[HuijingHei]

so this should not change in F43 (and C10S, and below).

Since we already do the moving for F43 and C10S, is it risky to roll back?
The latest bootupd does not support the old location /usr/lib/bootupd/updates/EFI, should we make it compatible? Or maybe rely on bootc-dev/bootc#1816 ?