I assume problem source is correction of bug https://bugzilla.opensuse.org/show_bug.cgi?id=1164385 "RPi3: no grub menu on serial console" with pr https://build.opensuse.org/request/show/784300 that added runtime check. Rather than to make changes in petitboot firmware, I would suggest to modify grub2.spec to not install /etc/grub.d/95_textmode for PowerPC.