- Added stm32f4 sdio driver, tested on stm32f4discovery.

- Improved the SDIO drivers for both stm32f1 and stm32f2. It now uses
  DMA transfers whenever possible. In the stm32f2 it is always possible,
  while in the stm32f1 it is done if the FSMC is not used for data storage.
- Added a test in the testsuite to check writing and reading large files
  from disk, with checksum of the written data.
- Modified the signature of crc16 to accept a void* instead of a char*,
  to avoid casting issues.