app/testpmd: add VXLAN encap/decap

app/testpmd: add VXLAN encap/decap

Due to the complex VXLAN_ENCAP flow action and based on the fact testpmd
does not allocate memory, this patch adds a new command in testpmd to
initialise a global structure containing the necessary information to
make the outer layer of the packet.  This same global structure will
then be used by the flow command line in testpmd when the action
vxlan_encap will be parsed, at this point, the conversion into such
action becomes trivial.

This global structure is only used for the encap action.

Signed-off-by: Nelio Laranjeiro <nelio.laranjeiro@6wind.com>
Acked-by: Ori Kam <orika@mellanox.com>
Acked-by: Adrien Mazarguil <adrien.mazarguil@6wind.com>
Acked-by: Mohammad Abdul Awal <mohammad.abdul.awal@intel.com>
Tested-by: Mohammad Abdul Awal <mohammad.abdul.awal@intel.com>