examples/l2fwd: add forwarding port mapping option

examples/l2fwd: add forwarding port mapping option

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

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

If no portmap 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 portmap
option is specified.

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

With above portmap 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: Vamsi Attunuru <vattunuru@marvell.com>
Signed-off-by: Pavan Nikhilesh <pbhagavatula@marvell.com>
Acked-by: Andrzej Ostruszka <aostruszka@marvell.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>