The main thing is to change the DMA code in a way that the structure
DMA_Stream_TypeDef (which is similar to DMA_Channel_TypeDef on stm32l4)
is no longer used outside of dma.c, as this structure only exists for the
F4 series.  Therefore I introduced a new structure (dma_descr_t) which
handles all DMA specific stuff for configuration.  Further the periphery
(spi, i2c, sdcard, dac) does not need to know the internals of the dma.