Ignore --uefi option for isohybrid on 32bit systems

This avoids this misleading error message:

| * Creating hybrid ISO file with isohybrid method
| * Detected uefi support for isohybrid, enabling
| isohybrid: /srv/jenkins/jobs/grml32-full_testing/workspace/grml_isos/grml32-full_testing_20120603.iso: unable to find efi image
|                                                                          [ !! ]