SPI DMA support

[thomasfla]

SPI transfer is quite slow with a delay between each byte of about
1.4us when calling SPI.transfer(uint8* buff, int N); with a 9Mhz SPI clock.

This delay is more than the duration of a byte transfer at this speed..

I think using DMA should solve this issue, but the current SPI library doesn't allow it.

Any chance this could be added to the todo features?

[fpistm]

They are already some improvement requests for SPI.
DMA could be one of this. Anyway I will not have time soon to do this. Any help are welcome

[VictorLamoine]

This should help you get started: http://stm32duino.com/viewtopic.php?f=14&t=3527&p=47009#p47009

[fpistm]

Close this one to track all SPI request on the same issue #257.
Mark it as duplicated.