Do not override the existing file system image on OTA

See: esp8266/Arduino#6505

Author: Lauszus

platformio.ini

@@ -16,13 +16,15 @@ include_dir = ./
 platform = espressif8266
 board = esp12e
 framework = arduino
-build_flags = -Wl,-Tesp8266.flash.4m1m.ld
 monitor_speed = 115200
 lib_deps = AsyncMqttClient
            ESP Async WebServer
            ArduinoJson
            AsyncElegantOTA
 
+# See: https://github.com/esp8266/Arduino/pull/6505
+build_flags = -Wl,-Tesp8266.flash.4m1m.ld -DATOMIC_FS_UPDATE
+
 ; Basic commands:
 ; pio run
 ; pio run -t upload