[romprefix] Force PnP header to a 16-byte boundary for IBM BIOSes

IBM BIOSes ignore the PnP header offset stored at address 0x1a and
instead scan for the $PnP signature on a 16-byte boundary.  (This
alignment is not mandated by the PnP specification.)

Force PnP header to a 16-byte boundary to work around these BIOSes.

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

src/arch/i386/prefix/romprefix.S

@@ -104,6 +104,11 @@ pciheader_runtime_length:
 	.long	0
 	.previous
 
+	/* PnP doesn't require any particular alignment, but IBM
+	 * BIOSes will scan on 16-byte boundaries rather than using
+	 * the offset stored at 0x1a
+	 */
+	.align	16
 pnpheader:
 	.ascii	"$PnP"			/* Signature */
 	.byte	0x01			/* Structure revision */