optional RTC 3231 integration

include/rtc_ds3231.h

@@ -0,0 +1,8 @@
+#pragma once
+
+#include <Arduino.h>
+
+bool rtc_ds3231_init();
+bool rtc_ds3231_is_present();
+bool rtc_ds3231_read_epoch(uint32_t &epoch_utc);
+bool rtc_ds3231_set_epoch(uint32_t epoch_utc);