Comment # 5 on bug 1157019 from Fabian Vogt

KIWI meanwhile uses what grub2-mkconfig produces and it's not really possible
to change that, other than mounting the image afterwards and writing into
/boot. Such a hack would only work until the first initrd rebuild as then the
config is regenerated.

So the right (and only) way to fix this is in the grub package or another
package which adds some stuff into /etc/grub.d/.