netperf: enable demo mode

Enable demo mode for netperf to activates a global "-D" <interval> option.
This option will display interim results at least every time interval.

Signed-off-by: Kelvin Cheung <keguang.zhang@gmail.com>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>