examples/l2fwd: add promiscuous mode option

examples/l2fwd: add promiscuous mode option

The default behaviour of l2fwd is to exit if we are unable to turn
promiscuous mode on. On some aws instances turning promiscuous mode
on is not permitted. In such cases there should be a way to run the
application without promiscuous mode.

This patch allows user to turn promiscuous mode on via command line
parameter. l3fwd has a similar option available.

Signed-off-by: Sarosh Arif <sarosh.arif@emumba.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>