sdl_mixer: run CONFIG_UPDATE on correct directory

The Makefile tried to update config.guess and config.sub in the wrong
directories, this patch updates the correct scripts making it possible to
compile on new systems.

Signed-off-by: Hans-Christian Egtvedt <hans-christian.egtvedt@atmel.com>

package/sdl_mixer/sdl_mixer.mk

@@ -16,7 +16,7 @@ sdl_mixer-source: $(DL_DIR)/$(SDL_MIXER_SOURCE)
 
 $(SDL_MIXER_DIR)/.unpacked: $(DL_DIR)/$(SDL_MIXER_SOURCE)
 	$(SDL_MIXER_CAT) $(DL_DIR)/$(SDL_MIXER_SOURCE) | tar -C $(BUILD_DIR) $(TAR_OPTIONS) -
-	$(CONFIG_UPDATE) $(SDL_MIXER_DIR)
+	$(CONFIG_UPDATE) $(SDL_MIXER_DIR)/build-scripts
 	touch $@
 
 $(SDL_MIXER_DIR)/.configured: $(SDL_MIXER_DIR)/.unpacked $(STAGING_DIR)/usr/lib/libSDL.so