$config_schema not working with a Configuration file.

I noticed this with a configuration file that stores the settings in an array, not a variable. This type of file uses the array_name array. The problem is in function restore_config. The function does not look in the array_name array to retrieve the config setting. My fix handles both types of configuration files.

I also added instructions in convert_phpbb20.php so users will know how to set up the $config_schema array.