examples/l2fwd-event: add option to configure port pairs

examples/l2fwd-event: add option to configure port pairs

Current l2fwd-event application statically configures adjacent ports as
destination ports for forwarding the traffic.

Add a config option to pass the forwarding port pair mapping which allows
the user to configure forwarding port mapping.

If no config argument is specified, destination port map is not
changed and traffic gets forwarded with existing mapping.

To align port/queue configuration of each lcore with destination port
map, port/queue configuration of each lcore gets modified when config
option is specified.

Ex: ./l2fwd-event -c 0xff -- -p 0x3f -q 2 --config="(0,3)(1,4)(2,5)"

With above config option, traffic received from portid = 0 gets forwarded
to port = 3 and vice versa, similarly traffic gets forwarded on other port
pairs (1,4) and (2,5).

Signed-off-by: Pavan Nikhilesh <pbhagavatula@marvell.com>
Reviewed-by: Andrzej Ostruszka <aostruszka@marvell.com>