app/testpmd: add csum parse-tunnel command

app/testpmd: add csum parse-tunnel command

Add a new command related to csum forward engine:

  csum parse-tunnel (on|off) (tx_port_id)

If enabled, the tunnel packets received by the csum forward engine are
parsed and seen as "outer-headers/inner-headers/data".

If disabled, the parsing of the csum forward engine stops at the first
l4 layer. A tunnel packet is seens as "headers/data" (inner headers are
included in payload).

Note: the port argument is the tx_port. It's more coherent compared
to all other testpmd csum flags.

Signed-off-by: Olivier Matz <olivier.matz@6wind.com>
Acked-by: Jijiang Liu <jijiang.liu@intel.com>