PowerPC fixes and enchancements

This set of commits brings grub2 for powerpc images
Can we change the control flow

```
if [ ! -z "$kiwi_OfwGrub" ];then
    # install powerpc grub2 
else
   if [ $isEFI -eq 0 ];then
      # use plain grub2-install in standard bios mode
   else
      # force install of grub2 in efi legacy mode
    fi
fi
```