There are some use cases that rely on having BOOT_IMAGE= set. There is a inconsistency on the s390x platform, because booting from DASD sets it, but booting from virtio disk in KVM does not. When looking at other platforms, then grub2 sets it on x86, aarch64, pseries (ppc).

Please read
https://bugzilla.redhat.com/show_bug.cgi?id=1782026 (the original request)
https://lists.gnu.org/archive/html/qemu-devel/2019-12/msg02999.html (follow up discussion with virt people)
for more information.