Like I said, I think Yoda's perfectly fine without backstory. Just think if we had to spend a prequel trying to explain where Yoda came from. There's enough needless exposition going on there as it is without that dragging it down. It just would have turned out horribly, and given us one more thing to complain about because we didn't agree where George was going/thought it ruined the character. Yoda is Yoda. That's all he needs to be. I've never cared what he is or where he comes from. The character stands on its own without any need for expository information.