Add support for ISO boot via systemd-boot

[schaefi]

Create proper EFI FAT image via bootctl to be used as alt loader in xorriso. This allows to boot the ISO via EFI e.g kvm -bios /usr/share/qemu/ovmf-x86_64.bin -cdrom file.iso Please note, hybrid boot is done via grub's hybrid MBR and as systemd-boot does not provide one, hybrid boot is out of scope yet. This Fixes #2281

[schaefi]

To build a live ISO image that boots via systemd perform the following actions

git checkout systemd_iso_support
tox -e unit_py3_11
. .tox/unit_py3_11/bin/activate
sudo ./.tox/unit_py3_11/bin/kiwi-ng --profile SDBoot system build --description build-tests/x86/tumbleweed/test-image-live --set-repo http://download.opensuse.org/tumbleweed/repo/oss --target-dir /tmp/mytest --allow-existing-root

To boot the resulting image call

kvm -bios /usr/share/qemu/ovmf-x86_64.bin -cdrom /tmp/mytest/kiwi-test-image-live.x86_64-1.42.3.iso

[schaefi]

So far so good but this PR has a major issue, which is currently documented in the following NOTE:

# NOTE: limitation of El Torito boot,
# if the ESP size is greater than 20MB, the machine
# does no longer boot. I could not find a solution to
# this issue and bootctl install + kernel-install needs
# ~200MB for the ESP. I also noted that kernel-install
# does not fail with an exit code != if there is no
# space left anymore. Thus the image build completes but
# with incomplete data on the ESP

[schaefi]

due to this issue we cannot merge this pull request and further investigations are needed if it is at all possible to include an embedded EFI partition of this size as El Torito boot. I got the information that systemd-boot will not gain native support for reading from an ISO9660 filesystem like e.g grub can do. Thus I'm hitting a limitation for which I don't have a good solution at the moment.

[schaefi]

I opened the PR for review as I think it can be merged because it fixes the systemd-boot setup in kiwi and also allows building bootable ISOs with systemd. The limitation of the ESP size is set on El Torito boot-load-size and I added a runtime check to check this condition such that users don't run into issues.

This still leaves the problem that the systemd-boot ESP grows beyond the size limitation of El Torito loader entries but it should not hold us back from merging the changes proposed to kiwi here.

Thanks