dfu: add scsi backend

This is extremely similar to the MMC backend, but there are some notable

Works with a DFU string like

    scsi 4=u-boot-bin part 11

Where "4" is the SCSI dev number (sequential LUN across all SCSI devices)
and "11" is the partition number.

Signed-off-by: Caleb Connolly <caleb.connolly@linaro.org>
5 files changed