Add WiSun network size configs in JSON

Author: fredlee12001

features/nanostack/mbed-mesh-api/mbed_lib.json

@@ -120,6 +120,10 @@
             "help": "default network name for wisun network",
             "value": "\"Wi-SUN Network\""
         },
+        "wisun-network-size": {
+            "help": "Expected amount of devices in the network as 100s of devices. with possible pre defined constants NETWORK_SIZE_SMALL, NETWORK_SIZE_MEDIUM, NETWORK_SIZE_LARGE, NETWORK_SIZE_XLARGE. if set to 0 Wi-SUN Certification configuration values are used. If don't define this(default null), then NETWORK_SIZE_MEDIUM will be used.",
+            "value": null
+        },
         "wisun-regulatory-domain": {
             "help": "Regulator domain.",
             "value": "3"

features/nanostack/mbed-mesh-api/source/WisunInterface.cpp

@@ -117,6 +117,14 @@ nsapi_error_t WisunInterface::configure()
     }
 #endif
 
+#ifdef MBED_CONF_MBED_MESH_API_WISUN_NETWORK_SIZE
+    status = set_network_size(MBED_CONF_MBED_MESH_API_WISUN_NETWORK_SIZE);
+    if (status < 0) {
+        tr_error("Failed to set network size");
+        return NSAPI_ERROR_PARAMETER;
+    }
+#endif
+
     return NSAPI_ERROR_OK;
 }