How to do "override def rootProject" in build.sbt?

[benmccann]

http://stackoverflow.com/questions/22216313/specifying-rootproject-in-build-sbt/22240142

In my Build.scala, I have:

override def rootProject = Some(frontendProject)

I'm trying to convert to the newer build.sbt format, but don't know the equivalent of this line. How do I set the project for sbt to load by default when using build.sbt?

Thanks,

-Ben