dist/tools/genconfig: add error on hidden symbols
dist/tools/genconfig: add error on hidden symbols

This extends the error messages to check if a symbol could not be set
due to its visibility.

jasonatran/RIOT,miri64/RIOT,RIOT-OS/RIOT,jasonatran/RIOT,OlegHahm/RIOT,kaspar030/RIOT,kaspar030/RIOT,kYc0o/RIOT,kYc0o/RIOT,kYc0o/RIOT,jasonatran/RIOT,ant9000/RIOT,kaspar030/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,OlegHahm/RIOT,kaspar030/RIOT,ant9000/RIOT,ant9000/RIOT,kYc0o/RIOT,OlegHahm/RIOT,RIOT-OS/RIOT,miri64/RIOT,RIOT-OS/RIOT,OlegHahm/RIOT,kYc0o/RIOT,OlegHahm/RIOT,miri64/RIOT,jasonatran/RIOT,ant9000/RIOT,miri64/RIOT,RIOT-OS/RIOT,ant9000/RIOT,miri64/RIOT