Doing cycling barebox boot over the net is a good testing practice.
It is possible to detect at least some bugs before flushing it to
the system. Suddenly it looks like, it is done not oft enough. So, let's
add this option mainline.

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>