dfu: add scsi backend

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

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>
diff --git a/drivers/dfu/Kconfig b/drivers/dfu/Kconfig
index 4e80e85..44e8d56 100644
--- a/drivers/dfu/Kconfig
+++ b/drivers/dfu/Kconfig
@@ -86,6 +86,13 @@
 	  used at board level to manage specific behavior
 	  (OTP update for example).
 
+config DFU_SCSI
+	bool "SCSI flash back end for DFU"
+	help
+	  This option enables using DFU to read and write to SCSI devices
+	  used at board level to manage specific behavior
+	  (OTP update for example).
+
 config SET_DFU_ALT_INFO
 	bool "Dynamic set of DFU alternate information"
 	help