dfu:cmd: Support for DFU u-boot command

Support for u-boot's command line command "dfu <interface> <dev> [list]".

Signed-off-by: Lukasz Majewski <l.majewski@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Cc: Marek Vasut <marex@denx.de>