examples/l2fwd: add option --[no-]mac-updating

examples/l2fwd: add option --[no-]mac-updating

l2fwd could be useful for testing virtual devices without the need
of physical ones.

To achieve this, this patch adds a new option to enable/disable the
MAC addresses updating done at forwarding time: --[no-]mac-updating

It enables the use of l2fwd for basic VM to VM communication.

By default, MAC address updating remains enabled, to keep consistency
with previous usage.

Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>
Acked-by: John McNamara <john.mcnamara@intel.com>