Hmm, to be honest I don't have any ideas other than it's indeed something with the basetools, but I'm not sure why it doesn't build for you. Any chance you could try a 64-bit Ubuntu to rule out at least this piece with a known problem on 32-bit ones?
By the way, which version of Ubuntu you're using? 12.04 is highly recommended versus any newer ones (at least for the Yocto part, not sure if that applies to the firmware part).