samples/net/tftp_client: Move from native_posix to native_sim

* In the sample doc, replace native_posix references with native_sim
* Provide an overlay for native_sim
* Enable native_sim in the test yaml

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>

samples/net/tftp_client/README.rst

@@ -41,15 +41,15 @@ Build the tftp-client sample application like this:
    :compact:
 
 The easiest way to setup this sample application is to build and run it
-as native POSIX application or as a QEMU target using the default configuration :file:`prj.conf`.
+as a native_sim application or as a QEMU target using the default configuration :file:`prj.conf`.
 This requires a small amount of setup described in :ref:`networking_with_eth_qemu`, :ref:`networking_with_qemu` and :ref:`networking_with_native_posix`.
 
-Build the tftp-client sample application for native_posix like this:
+Build the tftp-client sample application for :ref:`native_sim <native_sim>` like this:
 
 .. zephyr-app-commands::
    :zephyr-app: samples/net/tftp_client
    :host-os: unix
-   :board: native_posix
+   :board: native_sim
    :goals: run
    :compact:
 
@@ -70,7 +70,7 @@ configurations in ``prj.conf``::
 Sample output
 ==================================
 
-Sample run on native_posix platform with TFTP server on host machine
+Sample run on native_sim platform with TFTP server on host machine
 Launch net-setup.sh in net-tools
 .. code-block:: console
 

samples/net/tftp_client/boards/native_sim.conf

@@ -0,0 +1,3 @@
+CONFIG_DNS_RESOLVER=y
+CONFIG_DNS_SERVER_IP_ADDRESSES=y
+CONFIG_DNS_SERVER1="192.0.2.2"

samples/net/tftp_client/sample.yaml

@@ -7,8 +7,9 @@ tests:
     depends_on: netif
     platform_allow:
       - native_posix
+      - native_sim
     integration_platforms:
-      - native_posix
+      - native_sim
     tags:
       - net
       - tftp