We can't do it in the non-OSBuild configs because not all streams have the new coreos-installer yet.
Here is the comment from the original commit:
This setting will make it so that BLS config entries get prepended with /boot. OSTree already places a boot -> . symlink in the root of the boot filesystem prepending with /boot will always just work.
This is a second round of f5677a3 (reverted in db7bf08).
Now that coreos-installer in rawhide has https://github.com/coreos/coreos-installer/pull/1422 we can set sysroot.bootprefix to true again.
We can't do it in the non-OSBuild configs because not all streams have the new coreos-installer yet.
Here is the comment from the original commit:
This setting will make it so that BLS config entries get prepended with /boot. OSTree already places a boot -> . symlink in the root of the boot filesystem prepending with /boot will always just work.
For context see https://github.com/osbuild/osbuild/issues/1566#issuecomment-1921908500
This also allows for dropping one of the upstream OSBuild zipl stage patches.