MFC r354746:

MFC r354746:
loader: add support for hybrid PMBR for GPT partition table

Note hybrid table is nor really UEFI specification compliant.

Sample hybrid partition table:
> ::mbr
Format: unknown
Signature: 0xaa55 (valid)
UniqueMBRDiskSignature: 0

PART TYPE                  ACTIVE  STARTCHS    ENDCHS      SECTOR     NUMSECT
0    EFI_PMBR:0xee         0       1023/254/63 1023/254/63 1          409639
1    0xff                  0       1023/254/63 1023/254/63 409640     978508408
2    FDISK_EXT_WIN:0xc     0       1023/254/63 1023/254/63 978918048  31250000
3    0xff                  0       1023/254/63 1023/254/63 1010168048 32
>