ReactOS fails to boot if the BIOS supplies an incorrect boot drive

Some BIOSes supply either an incorrect boot drive number or use a numbering scheme that is
incompatible with what the BIOS int13 interface expect (e.g. 1 for first floppy, 2 for second floppy,
3 for first harddisk and so on). Since the MBR code from ReactOS relies entirely on the BIOS supplying
a correct value in the sheme expected by the int13 interface, the system fails to boot in such a case.

Could in theory worked around by e.g. storing the original drive number in the boot record and use
that instead (or some hybrid solution), but may probably impact the ability to boot from any drive number.

Thanks to oldman for bringing this up in the forum and testing against the suspected source of the problem.