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>