ethdev: add tunnel encap/decap actions

ethdev: add tunnel encap/decap actions

Add new flow action types and associated action data structures to
support the encapsulation and decapsulation of VXLAN and NVGRE tunnel
endpoints.

The RTE_FLOW_ACTION_TYPE_[VXLAN/NVGRE]_ENCAP action will cause the
matching flow to be encapsulated in the tunnel endpoint overlay
defined in the [vxlan/nvgre]_encap action data.

The RTE_FLOW_ACTION_TYPE_[VXLAN/NVGRE]_DECAP action will cause all
headers associated with the outer most tunnel endpoint of the specified
type for the matching flows.

Signed-off-by: Declan Doherty <declan.doherty@intel.com>