Update gen_release to support regular automated builds.

Updates to support building tarballs more regularly (e.g. after each push to master).

Update gen_release to work inside existing repository -- the automated build system
already ensures a pristine working copy. Add new bash script in util/cron/ that calls
gen_release with non-clone settings and a version number like `1.10.0.deadbee`.

Also, update `create_release_tarball.bash` script to use new non-clone setting.