SaveData: Implemented the SystemSaveData archive.

It will be stored in the /syssavedata folder. This archive is user by various Services and possibly games via the FS:U service.
SystemSaveData: Added a TODO to move it to the NAND.

Maybe sometime when we actually implement that
Merge pull request #301 from Subv/more_savedata

SaveData: Implemented the SystemSaveData archive.