uboot-phytec

dfu: Refactor do_dfu() to handle optional argument

In the future we may utilize optional argument in 'dfu' command line.
As a preparation for this, refactor do_dfu().

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Lukasz Majewski <lukma@denx.de>