Package order options should always write a path to "PreOS-Packages" folder

The config.plist file only gets a reference to "PreOS-Packages" if a package in the workflow is switched from after to before. It would be better if the reference to the "PreOS-Packages" path was always written to config.plist in order to let the user drag packages manually there without needing to check if config.plist currently had a reference to the location.