dfu: command: Extend "dfu" command to handle receiving data via TFTP

The "dfu" command has been extended to support transfers via TFTP protocol.

Signed-off-by: Lukasz Majewski <l.majewski@majess.pl>
Reviewed-by: Simon Glass <sjg@chromium.org>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>