doc: net: Add instructions for native_posix board

Add generic instructions how to use native_posix board with
networking enabled.

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

doc/subsystems/networking/native_posix_setup.rst

@@ -0,0 +1,87 @@
+.. _networking_with_native_posix:
+
+Networking with native_posix board
+##################################
+
+This page describes how to set up a virtual network between a (Linux) host
+and a Zephyr application running in a native_posix board.
+
+In this example, the :ref:`echo-server-sample` sample application from
+the Zephyr source distribution is run in native_posix board. The Zephyr
+native_posix board instance is connected to a Linux host using a tuntap device
+which is modeled in Linux as an Ethernet network interface.
+
+Prerequisites
+*************
+
+On the Linux Host, fetch the Zephyr ``net-tools`` project, which is located
+in a separate Git repository:
+
+.. code-block:: console
+
+   git clone https://github.com/zephyrproject-rtos/net-tools
+
+
+Basic Setup
+***********
+
+For the steps below, you will need three terminal windows:
+
+* Terminal #1 is terminal window with net-tools being the current
+  directory (``cd net-tools``)
+* Terminal #2 is your usual Zephyr development terminal,
+  with the Zephyr environment initialized.
+* Terminal #3 is the console to the running Zephyr native_posix
+  instance (optional).
+
+Step 1 - Create Ethernet interface
+==================================
+
+Before starting native_posix with network emulation, a network interface
+should be created.
+
+In terminal #1, type:
+
+.. code-block:: console
+
+   ./net-setup.sh
+
+You can tweak the behavior of the net-setup.sh script. See various options
+by running ``net-setup.sh`` like this:
+
+.. code-block:: console
+
+   ./net-setup.sh --help
+
+
+Step 2 - Start app in native_posix board
+========================================
+
+Build and start the ``echo_server`` sample application.
+
+In terminal #2, type:
+
+.. zephyr-app-commands::
+   :zephyr-app: samples/net/echo_server
+   :host-os: unix
+   :board: native_posix
+   :goals: run
+   :compact:
+
+
+Step 3 - Connect to console (optional)
+======================================
+
+The console window should be launched automatically when the Zephyr instance is
+started but if it does not show up, you can manually connect to the console.
+The native_posix board will print a string like this when it starts:
+
+.. code-block:: console
+
+   UART connected to pseudotty: /dev/pts/5
+
+You can manually connect to it like this:
+
+.. code-block:: console
+
+   screen /dev/pts/5

doc/subsystems/networking/networking.rst

@@ -20,6 +20,7 @@ operation of the stacks and how they were implemented.
    net-app-api.rst
    buffers.rst
    qemu_setup.rst
+   native_posix_setup.rst
    usbnet_setup.rst
    vlan.rst
    traffic-class.rst