[image] Skip misleading "format not recognised" error message

Return success (rather than failure) after an image format has been
correctly identified.

This has no practical effect, since the return value from
image_probe() is deliberately never used, but avoids a somewhat
surprising and misleading "format not recognised" error message when
debugging is enabled.

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

src/core/image.c

 			image->type = type;
 			DBGC ( image, "IMAGE %s is %s\n",
 			       image->name, type->name );
-			break;
+			return 0;
 		}
 		DBGC ( image, "IMAGE %s is not %s: %s\n", image->name,
 		       type->name, strerror ( rc ) );