ethdev: support L2 tunnel operations

ethdev: support L2 tunnel operations

Add functions to support l2 tunnel configuration and operations.
1, L2 tunnel ether type modification.
   It means modifying the ether type of a specific type of tunnel.
   So the packet with this ether type will be parsed as this type
   of tunnel.
2, Enabling/disabling l2 tunnel support.
   It means enabling/disabling the ability of parsing the specific
   type of tunnel. This ability should be enabled before we enable
   filtering, forwarding, offloading for this specific type of
   tunnel.
3, Insertion and stripping for l2 tunnel tag.
4, Forwarding the packets to a pool based on l2 tunnel tag.

Only support e-tag tunnel now.

Signed-off-by: Wenzhuo Lu <wenzhuo.lu@intel.com>
Acked-by: Shaopeng He <shaopeng.he@intel.com>
Acked-by: Jingjing Wu <jingjing.wu@intel.com>
Tested-by: Yong Liu <yong.liu@intel.com>