new support for Raspberry Pi B+

[draymond at FoxValley.net]

> It seems that "portsnap fetch update" is a consistent way to  
> generate  a panic.  I have now seen panics with all four of my SD  
> cards on two  different Raspberry Pis, and with three different  
> power cables.  All  occurred while running at low speed for SD  
> (25MHz) on r274416.
>
> ...
>
> Ian, you mentioned that you thought this looked like a memory   
> corruption similar to the issues reported on Wandboard.  I have been  
>  reading those threads but I don't fully understand what is the  
> issue.   Can you clarify?  I also saw some discussion about some new  
> changes  currently under testing and planned for release.  Are these  
> expected  to resolve the memory corruption?  What is the root cause  
> and is the  problem present in all builds or just recent builds?

Ian, can you comment?