Some MQTT updates

Change /pong to /uptime
Display a message if MQTT_SERVER is not defined
Put picow_ in the name like the other examples
Add details to the readme!

README.md

@@ -191,6 +191,7 @@ App|Description
 [picow_httpd](pico_w/wifi/httpd) | Runs a LWIP HTTP server test app
 [picow_http_client](pico_w/wifi/http_client) | Demonstrates how to make http and https requests
 [picow_http_client_verify](pico_w/wifi/http_client) | Demonstrates how to make a https request with server authentication
+[picow_mqtt_client](pico_w/wifi/mqtt) | Demonstrates how to implement an MQTT client application
 
 #### FreeRTOS examples
 

pico_w/wifi/mqtt/CMakeLists.txt

@@ -6,6 +6,7 @@ if (DEFINED ENV{MQTT_SERVER} AND (NOT MQTT_SERVER))
     message("Using MQTT_SERVER from environment ('${MQTT_SERVER}')")
 endif()
 if (NOT MQTT_SERVER)
+    message("Skipping MQTT example as MQTT_SERVER is not defined")
     return()
 endif()
 # Define the name of an MQTT broker/server to enable this example
@@ -32,7 +33,7 @@ if (NOT MQTT_CERT_INC)
     set(MQTT_CERT_INC mqtt_client.inc)
 endif()
 
-set(TARGET_NAME mqtt_client)
+set(TARGET_NAME picow_mqtt_client)
 add_executable(${TARGET_NAME}
     mqtt_client.c
     )

pico_w/wifi/mqtt/mqtt_client.c

@@ -206,7 +206,7 @@ static void mqtt_incoming_data_cb(void *arg, const u8_t *data, u16_t len, u8_t f
     } else if (strcmp(basic_topic, "/ping") == 0) {
         char buf[11];
         snprintf(buf, sizeof(buf), "%u", to_ms_since_boot(get_absolute_time()) / 1000);
-        mqtt_publish(state->mqtt_client_inst, full_topic(state, "/pong"), buf, strlen(buf), MQTT_PUBLISH_QOS, MQTT_PUBLISH_RETAIN, pub_request_cb, state);
+        mqtt_publish(state->mqtt_client_inst, full_topic(state, "/uptime"), buf, strlen(buf), MQTT_PUBLISH_QOS, MQTT_PUBLISH_RETAIN, pub_request_cb, state);
     } else if (strcmp(basic_topic, "/exit") == 0) {
         state->stop_client = true; // stop the client when ALL subscriptions are stopped
         sub_unsub_topics(state, false); // unsubscribe