The system (HP Compaq 6200 Pro MT PC) was running with OS 13.2 and a working
grub bootloader. I did an upgrade to 42.2 via DVD. The upgrade failed when
installing the bootloader with
an shim error ("EFI Partition not found" ? - not sure).

I was able too boot the system via DVD and did an update to the latest
versions.
This did not fix the problem neither changing the bootloader to non-efi grub.
After various attempts i realized that there was no /boot/efi/... what so ever.
Coping that from DVD made the system bootable again.

A good indicator was that grub2-mkconfig showed an error (missing /boot/efi/..)
in the XEN part. I ignored that in the beginning as i do not use XEN.

Suggestion:
* a more descriptive error for shim ("Partition" let me search with fdisk)
* double check that /boot/efi/.. and friends really, really exists
* Give clueless user a hint what to do when yast fails to install a bootloader
(like "use DVD to recover" ... )
* on error: offer to remove all traces from efi (like the opensuse entry) in
the vain hope that an other bootloader may be more successful when not confused
with leftovers