fix(disk_util): Switch back to gptmbr.bin for our MBR boot code.

Using the classic mbr.bin was only needed during the transition from
syslinux 3 to 6 because the behavior of gptmbr.bin changed after 3.
Now that the transition is done and cgpt supports the new scheme now it
is time we switched back. This avoids depending on using a hybrid MBR.
Merge pull request #233 from marineam/mbr

fix(disk_util): Switch back to gptmbr.bin for our MBR boot code.