pg: misc improvements and fixes

pg: misc improvements and fixes

1) pg can typically injects packets in ethernet-input, ip4-input or
ip6-input. Make sure offload offsets are correctly set for ip4-input and
ip6-input.
2) add hw-addr support for ethernet mode (only available through cli)
3) refactor pg creation code to improve the readability by using
data structure pg_interface_args_t
4) fix the pg input and output traces to use headers according to
pg interface mode
5) introduce pg interface flags i.e. checksum, gso, gro

Type: improvement

Change-Id: Iffed502e9c6357d7ef8e8a72217867e8297236aa
Signed-off-by: Benoît Ganne <[email protected]>
Signed-off-by: Mohsin Kazmi <[email protected]>