[int13] Include disk signature in debugging output

The disk signature is used by some OSes (notably Windows) to identify
the boot disk, so it's useful debugging information to have.

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

src/arch/i386/include/int13.h

@@ -249,11 +249,16 @@ struct partition_table_entry {
 
 /** A Master Boot Record */
 struct master_boot_record {
-	uint8_t pad[446];
+	/** Code area */
+	uint8_t code[440];
+	/** Disk signature */
+	uint32_t signature;
+	/** Padding */
+	uint8_t pad[2];
 	/** Partition table */
 	struct partition_table_entry partitions[4];
 	/** 0x55aa MBR signature */
-	uint16_t signature;
+	uint16_t magic;
 } __attribute__ (( packed ));
 
 /** Use natural BIOS drive number */

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

@@ -368,6 +368,8 @@ static int int13_guess_geometry ( struct int13_drive *int13 ) {
 		       int13->drive, strerror ( rc ) );
 		return rc;
 	}
+	DBGC ( int13, "INT13 drive %02x has signature %08x\n",
+	       int13->drive, mbr.signature );
 
 	/* Scan through partition table and modify guesses for heads
 	 * and sectors_per_track if we find any used partitions.