feat(disk_util): Switch from SYSLINUX's gptmbr.bin to mbr.bin

cgpt now supports generating hybrid MBRs and the classic style mbr.bin
from any version of SYSLINUX should work the same with the hybrid MBR.
The other code, gptmbr.bin, changes after SYSLINUX 3. Switching lets me
play with different versions of SYSLINUX without breaking everything.

With this change all images feature a hybrid MBR so the special case for
some VM platforms has been removed.
Merge pull request #188 from marineam/syslinux

feat(disk_util): Switch from SYSLINUX's gptmbr.bin to mbr.bin