samples: net: lwm2m_client: Use cert-based DTLS overlay in twister

Make sure that certificate-based DTLS config is built by twister and
also mention it in the README file.

Signed-off-by: Robert Lubos <[email protected]>

Author: rlubos

samples/net/lwm2m_client/README.rst

@@ -49,6 +49,9 @@ samples/net/lwm2m_client directory:
     * - :file:`overlay-dtls.conf`
       - This overlay config can be added for DTLS support via MBEDTLS.
 
+    * - :file:`overlay-dtls-cert.conf`
+      - This overlay config can be added for DTLS with certificates support via MBEDTLS.
+
     * - :file:`overlay-queue.conf`
       - This overlay config can be added to enable LWM2M Queue Mode support.
 

samples/net/lwm2m_client/sample.yaml

@@ -52,6 +52,18 @@ tests:
     tags:
       - net
       - lwm2m
+  sample.net.lwm2m_client.dtls_cert:
+    harness: net
+    depends_on: netif
+    extra_args: EXTRA_CONF_FILE=overlay-dtls-cert.conf
+    platform_allow:
+      - qemu_x86
+      - native_sim
+    integration_platforms:
+      - qemu_x86
+    tags:
+      - net
+      - lwm2m
   sample.net.lwm2m_client.queue_mode:
     harness: net
     depends_on: netif