At the core of Yocto is the Poky build system, and from a developers point of view, this is what we work with - a poky git repository. Poky provides several example Linux images, including minimal, sato, and sdk, each with an increasing number of packages (and footprint size). Poky focuses on providing a small core set of known-working packages, if you want to add to that, it offers layers which work sort of like a build-system version of external distribution package repositories (world, universe, and the like).
Yocto is targetted at deeply embedded devices. In addition to supporting multiple architectures, it is our goal to support different kernel types to meet the varying needs of embedded projects. To this end, I have put together two Yocto layers: meta-linaro and meta-rt.
The meta-linaro layer incorporates a new kernel recipe using the stable Linaro kernel sources as well as a new toolchain recipe (by Nitin Gamble) using the Linaro gcc sources. I've posted a meta-linaro layer to the new poky-extras repository:
The meta-rt layer leverages the internal meta-data of the linux-yocto kernel repository to build a 2.6.34 based PREEMPT_RT kernel. It adds a new recipe for the rt-tests real-time test suite (including well known tests such as cyclictest) and a poky-linux-minimal-rt image to bundle them together. It is boot tested on qemux86-64 at the moment, with atom-pc and additional architectures to follow shortly. The work-in-progress PREEMPT_RT layer is available here:
If you're interested in Linaro or real-time, give them a spin and let us know how it goes.