Kconfig: Add option to toggle app's function from publisher to subscriber

When CONFIG_MQTT_SUBSCRIBER=y the application should switch
modes from being a publisher on the topic to being a subscriber
on that topic.

Signed-off-by: Zbigniew Bodek <zbigniew.bodek@huawei.com>

Kconfig

@@ -21,4 +21,11 @@ config NET_SAMPLE_APP_MAX_CONNECTIONS
 	  send NET_SAMPLE_APP_MAX_ITERATIONS amount of MQTT sample messages.
 	  A value of zero means to continue forever.
 
+config MQTT_SUBSCRIBER
+	bool "Switches main app function from publisher to subscriber"
+	help
+	  Switches main app function from publisher to subscriber.
+	  When changed to "true", the application will not publish
+	  on any topic but rather subscribe to the topic.
+
 source "Kconfig.zephyr"