[SYS] Password management improvements (#1787)

* Use one single password for Access point, OTA and WebUI access
* Enable to change the password during WM onboarding, with the WebUI, and MQTT command
* Remove the default password during the first configuration to connect to the AP
* When using the last 8 digits of the MAC as the password, it can now be changed
* Remove gateway password sending to client

docs/upload/builds.md

@@ -90,7 +90,7 @@ build_flags =
   '-DMQTT_USER="mqttusername"'
   '-DMQTT_PASS="mqttpassword"'
   '-DMQTT_SERVER="mqttserver.local"'
-  '-Dota_password="otapassword"'
+  '-Dgw_password="12345678"'
   '-DLED_RECEIVE=LED_BUILTIN'        ; Comment 1
   '-DLED_RECEIVE_ON=LOW'             ; Comment 2
   '-DRF_RECEIVER_GPIO=13'