Andreas: I thought this is about /sbin/installkernel from dracut.
There is:
case "$(uname -m)" in
s390|s390x)
BOOTFILE=image
;;
ppc|ppc64)
BOOTFILE=vmlinux
;;
*)
BOOTFILE=vmlinuz
;;
esac
If you need something specific for aarch64, please let me know or best send a
tiny patch which shouldn't be hard to do, but needs ARM HW at hand...