[int13] Dump out MBR at DBGLVL_EXTRA

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/arch/i386/interface/pcbios/int13.c

 		       int13->drive, strerror ( rc ) );
 		return rc;
 	}
+	DBGC2 ( int13, "INT13 drive %02x has MBR:\n", int13->drive );
+	DBGC2_HDA ( int13, 0, &mbr, sizeof ( mbr ) );
 	DBGC ( int13, "INT13 drive %02x has signature %08x\n",
 	       int13->drive, mbr.signature );