[gPXE] Only register bios drive without booting

[Miller, Shao]

Good day Andrew,

I believe that way it works is: gPXE checks the MBR for an MBR
signature.  gpxe/src/arch/i386/interface/pcbios/int13.c: int13_boot().
If that fails, control is returned to gPXE.  Also, if an MBR calls INT
0x18 or INT 0x19, I believe control is also returned to gPXE.

So a zeroed-out MBR should work, as you mention, and so should an MBR's
boot code finding no active partition and calling INT 0x18[2].

- Shao Miller

[1] http://www.etherboot.org/wiki/sanboot/iscsi_install
[2] http://www.ata-atapi.com/hiwmbr.html