Option saving/loading

[y21]

• Move all the update logic out of main into its own rrc_update_do_updates function. This is so that we can call it when "Perform updates" is pressed in the settings.
• Implement settingsfile.c which can load a settings file on the sd card into memory and save it again.
• Parse possible choices in RetroRewind6/xml/RetroRewind6.xml
• Some small refactoring/simplification in the settings code (we now store an options_count in the struct and no longer have a NULL at the end of the arrays, so we can replace some loops to find the array length by reading that count field directly)