* generate unique serial number (#90)
* see https://github.com/ricaun/ArduinoUniqueID
* calculated from ESP.getEfuseMac() on ESP32
* ESP.getChipId() on ESP8266
* SERIAL_NUMBER_WORD_0-3 on SAMD
* HAL_GetUIDw0-2() on STM32
* defaults to 0x01020304 on other platforms
* fix variable name for ESP platform
* another fix variable name for ESP platform (need more coffee...)