connman: allow user editable main.conf

In recent months I have seen a number of cases where users need to change connman's ‘prefer ethernet’ to ‘prefer wifi’ behaviour, or where users are trying to enable tethering for ethernet, or are running other daemons that add network interfaces that should not be auto-managed by comman. All of these require user editing of the connman main.conf file, so this change moves it to /storage where it can be tweaked if needed.

NB: I have not actually tested this as i'm in a hotel somewhere, but you get the idea :)