config: Cleanup .json config file generation

config: Cleanup .json config file generation

This patch 'completes' .json config files generation by adding in the
SimObject references and String-valued parameters not currently
printed.

TickParamValues are also changed to print in the same tick-value
format as in .ini files.

This allows .json files to describe a system as fully as the .ini files
currently do.

This patch adds a new function config_value (which mirrors ini_str) to
each ParamValue and to SimObject.  This function can then be explicitly
changed to give different .json and .ini printing behaviour rather than
being written in terms of ini_str.