Michael Haberler's new branch supports the Xenomai Realtime kernel. Information on installing it can be found here: NewRTInstall.
This is prerequisite to building the kernel itself.
The recipe is here: XenomaiUserlandPackage
There are a few ways to do this.
This is a good way to build a quick, one-off kernel package.
The recipe is here: XenomaiKernelPackageFromTarball
This is ideal for a kernel package maintainer who wants to provide sustained, updated kernel packages.
The recipe to get started is here: XenomaiKernelPackageFromUbuntuGit