grub2pxe takes this to mean boot through the MBR on legacy BIOS
machines or boot through the EFI partition for UEFI machines.
It has the interesting, unanticipated benefit that it means that
the BSD-based pxeboot will work for disk booting boot images on
legacy BIOS machines. It doesn't have to know anything about GPT
in this case, it just boots sector 0!

This is possibly the wrong place for this. Arguably, we should put
the 0 value in the partitions table of the DB and leave bootinfo
along, but I am not sure of what side-effects there might be doing
it that way.