samples: net: Add socket-based echo server example

The example source code is POSIX-compatible (modulo include files),
i.e. can be built and behaves the same way for Zephyr and a POSIX
system (e.g. Linux). Makefile.posix is available for the latter.

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>
5 files changed