app/testpmd: add parameter to start forwarding Tx first

app/testpmd: add parameter to start forwarding Tx first

Add parameter to start forwarding sending first
a burst of packets, which is useful when testing
a loopback connection.

This was already implemented as an internal command,
but adding it as a parameter is interesting, as it
allows the user to test a loopback connection without
entering in the internal command line.

Signed-off-by: Pablo de Lara <pablo.de.lara.guarch@intel.com>
Acked-by: Jingjing Wu <jingjing.wu@intel.com>