[LPM] Add capability to deactivate the function at build time (#1449)

To enable boards with the constant power supply to don't build this function.
Change the default behavior for the mode not to be built by default.
Only the boards with batteries are keeping it

main/ZmqttDiscovery.ino

@@ -1029,6 +1029,7 @@ void pubMqttDiscovery() {
                   stateClassNone, //State Class
                   "false", "true" //state_off, state_on
   );
+#      if DEFAULT_LOW_POWER_MODE != -1
   createDiscovery("switch", //set Type
                   subjectSYStoMQTT, "SYS: Low Power Mode command", (char*)getUniqueId("lowpowermode", "").c_str(), //set state_topic,name,uniqueId
                   "", "", "{{ value_json.lowpowermode | bool }}", //set availability_topic,device_class,value_template,
@@ -1039,6 +1040,7 @@ void pubMqttDiscovery() {
                   stateClassNone, //State Class
                   "false", "true" //state_off, state_on
   );
+#      endif
 #    endif
 #  endif
 }