[image] Automatically free autobooted images

Simplify the process of booting by ensuring that old images are not
left registered after an unsuccessful autoboot attempt.

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

src/usr/autoboot.c

 	if ( filename ) {
 		if ( ( rc = imgdownload ( filename, &image ) ) != 0 )
 			goto err_download;
+		image->flags |= IMAGE_AUTO_UNREGISTER;
 		if ( ( rc = image_exec ( image ) ) != 0 ) {
 			printf ( "Could not boot image: %s\n",
 				 strerror ( rc ) );