net: Add option for tracing packets

Add an option to trace all packets send/received. This can be helpful when
debugging protocol issues, as the packets can then be imported into
wireshark [1] and analyzed further.

[1] https://www.wireshark.org/docs/wsug_html_chunked/ChIOImportSection.html



Signed-off-by: Sean Anderson <seanga2@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>