mbr: Make sure the MBR code starts with the byte 0x33

Apparently some BIOSes (including some Acer Travelmate machines)
require an MBR to start with 0x33; apparently Micro$oft MBRs start
with 33 C0, an alternate coding of the "xorw %ax,%ax" instruction.  As
such, follow suit to work on these braindead BIOSes.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>
4 files changed