Refactor main.cpp

Pulled the Arduino setup() and loop() function into main() directly.
This allowed the elimination of most globals since the objects were now
available at local scope.
Pull config.h settings from priv_config.h if it exists

The makefile will now include priv_config.h before including any other
headers into all C++ source files.  This means that the settings in
config.h will be ignored and the developer's private settings will be
used instead.

I also added priv_config.h to the .gitignore so that a developer
doesn't check it in by mistake.

This allows each developer to have their own customized settings, such
as S_CALLSIGN and not have to worry about pushing them up to the main
git repo.