A useful default /etc/sysconfig/kernel is now shipped as part of grubby as of grubby-8.40-86, so this is not needed. Reference: https://src.fedoraproject.org/rpms/grubby/c/482a2825c47c4fd28a18beb279a3e826461bf878
This has been fixed properly with kiwi 9.25.21.