samples/net: Add a simple telnet sample

It will setup IPv4/IPv6 addresses. Telnet is ran in the background,
along with net and kernel shell modules.

See the README.rst for more information how to start and use this
sample.

Change-Id: I3e935014c79b534aab43a6fa8256792b23abb38e
Signed-off-by: Tomasz Bursztyka <tomasz.bursztyka@linux.intel.com>