Stm32f3 paparazzi

Some fixes, additions and changes to be prepared for the upcomming stm32f3discovery paparazzi support.

Fixed, changed, improved: i2c, dma, spi. Please look at individual commits for more info.

DMA changes may break some client code. I think the change makes sense because the f0 and the f3 dma units are basically the same and this avoids having to use #ifdefs for something that is clearly the same.