[embed] Add missing register_image() to image/embedded.c

When the embedded image is a script, the unregister_image() performed by
image/script.c corrupts memory, since image/embedded.c omitted the call
to register_image().

This is the first bug fixed using Stefan Hajnoczi's gdb stub for gPXE.

src/image/embedded.c

@@ -39,6 +39,7 @@ struct image *embedded_image(void)
 		return image = NULL;
 	}
 	copy_to_user(image->data, 0, _embedded_image_start, eisize);
+	register_image(image);
 
 	/* Reclaim embedded image memory */
 	reclaimed = 1;