pocket: central location for common configuration

[jotego]

Develop a core to set up common options to all cores. Particularly to support #696. This may be taken over to MiSTer too in order to support common CRT settings.

[jotego]

A central core that save settings in the Pocket cannot be made because the configuration file will not be accessible by the other cores. Only files in the common folder of a platform are readable by other cores.

We cannot make a central core, but we can still edit a file using an external tool and store the file in the right location so all cores can access it. This external tool can be provided by us but also third party developers (i.e. updater developers) can include this functionality.