net: apps: Add TCP support to echo-client application

If TCP is activated, then the echo-client application opens
a TCP socket and starts to send data to server. For TCP
connections, the application data is not reversed because
it would make it difficult to verify the data in the sender
side.

Change-Id: I35306b8ecd6794365086a23bf6a17f5103d7ac80
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>