Inspired by Tycho and Maven, I've simplified the self-bootstrapping process for Athena builds.
Previously, you needed a start.sh script to fetch org.eclipse.releng.basebuilder and org.eclipse.dash.common.releng before you could run a build; if on build.eclipse.org this is done for you automatically. Or, if building locally (or on Windows), you had to fetch these yourself.
Now, you can simply tell your build.xml to bootstrap itself, and it will fetch those projects for you. You can also tell it which version of basebuilder or common.releng to use, rather than being tied to the defaults.
For details, see Getting Started - Bootstrapping.