isohybrid: Generate MBR even when in EFI mode

isohybrid: Generate MBR even when in EFI mode

Various EFI systems insist that there be no active flag in the pMBR in
order to parse the GPT. The only way around this is to also generate a
valid MBR - the firmware will then pick that up and use the system
partition provided there. In order to deal with other EFI "sanity" checks,
the partition type for the non-EFI partitions is set to 0 to skip the
firmware bailing because of overlapping partitions.

Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>