examples/l3fwd: make Rx and Tx queue size configurable

examples/l3fwd: make Rx and Tx queue size configurable

Make Rx and Tx queue sizes configurable from the command line.
This helps DTS write better test cases.

Signed-off-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>
Reviewed-by: Kathleen Capella <kathleen.capella@arm.com>
Reviewed-by: Ruifeng Wang <ruifeng.wang@arm.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>