Syncronize local changes to thermite.sh:

 - In build_release(), use source_config() instead of
   continuing if the configuration file exists to prevent
   erroneous output from printenv(1).

 - In truncate_logs(), only create the log file if there is
   a corresponding configuration file for the build, otherwise
   log files that will never be used are created (such as
   11-amd64-GENERIC64-snap.log).

Sponsored by:	The FreeBSD Foundation