You need to have an RPM-based machine (Fedora/RHEL works) for this to work.
Install the createrepo package:
$ sudo yum install createrepo
Make sure you have the private GPG key to be allowed to sign.
Now make the repo:
$ ./build.sh
This will download any rpm file for Ceylon unless already present in the current folder,
sign them and create an RPM repo in repodata.