dfu: Add optional timeout parameter

When the `dfu` command is called from the U-Boot environment,
it now accepts an optional parameter that specifies a timeout (in seconds).
If a DFU connection is not made within that time the `dfu` command exits
(as it would if Ctrl+C was pressed). If the timeout is left empty or being
zero the `dfu` command behaves as it does now.

This is useful for allowing U-Boot to check to see if anything wants to
upload new firmware before continuing to boot.

The patch is based on the commit
https://github.com/01org/edison-u-boot/commit/5e966ccc3c65c18c9783741fa04e0c45e021780c
by Sebastien Colleur, which has been heavily reworked due to U-Boot changes
in the past.

Signed-off-by: Brad Campbell <bradjc5@gmail.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
diff --git a/drivers/dfu/Kconfig b/drivers/dfu/Kconfig
index 75fe0a1..9709b62 100644
--- a/drivers/dfu/Kconfig
+++ b/drivers/dfu/Kconfig
@@ -23,6 +23,12 @@
 
 	  Detailed description of this feature can be found at ./doc/README.dfutftp
 
+config DFU_TIMEOUT
+	bool "Timeout waiting for DFU"
+	help
+	  This option adds an optional timeout parameter for DFU which, if set,
+	  will cause DFU to only wait for that many seconds before exiting.
+
 config DFU_MMC
 	bool "MMC back end for DFU"
 	help