helper/configuration: free script_search_dirs and config_file_names

Although the leak is negligible, the clean heap on exit will ease
valgrind testing.

Change-Id: If43f02fe594c30ceb1bea3259ea3e098d4b2d239
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-on: http://openocd.zylin.com/4408
Tested-by: jenkins
Reviewed-by: Marc Schink <openocd-dev@marcschink.de>

src/helper/configuration.h

@@ -32,6 +32,8 @@ void add_config_command(const char *cfg);
 
 void add_script_search_dir(const char *dir);
 
+void free_config(void);
+
 int configuration_output_handler(struct command_context *cmd_ctx,
 		const char *line);