Yocto, RaspberryPi, LinuxCNC

Hello.

I try to gain understanding, how can I build a LinuxCNC image for RaspberryPi (using Yocto build system).
My idea is that the update process is then easy when once set up.
Would you give me guidance or pointers to information, how can I do that.
I have set up the yocto environment and the rpi-basic-image is so far the only image that I've succeeded to build.

I do not want to install the LinuxCNC into my desktop PC (Ubuntu 12.04 LTS).

So:
- what changes do I need to make RT image for RPi (on recipe level)
- what is the best way to add LinuxCNC on top of it