[Tinyos Core WG] TEP 107 and RealMainP update

[Philip Levis]

I've updated RealMainP to follow TEP 107, with a platform_bootstrap()  
function. This requires that each platform have a platform.h file so  
it can override the default. I put empty platform.h files in each  
platform directory and will document its presence in TEP 107.

I also checked in the changes to the atm128 SPI driver that fixes the  
0-length write bug.

Phil