Replace unnecessarily complicated Yocto build for BEEs

I chose Yocto to construct the first BEE images, mostly because I was expecting to have to fiddle with performance and power issues. I now believe that we only need the PREEMPT patches and that the chipset does not give any useful leverage to the OS to control power.

Yocto is surprisingly complicated, and makes building new images difficult. This just caused trouble at LAM, since the gatevalve (ADIO) kernel driver could not be built quickly.

So we should switch to something (much) simpler.