net: apps: Connect echo-server and echo-client tests together

It is now possible to loop echo-server and echo-client test
applications together when they are running in qemu.

Start echo-server with "make server" and echo-client using
"make client" in the corresponding samples directory.

Change-Id: Ic852735f7bebc93e42873e98427e282902b3ca03
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>