drivers/periph_common: RTC: move RIOT_EPOCH to header, document it.

drivers/include/periph/rtc.h

@@ -46,6 +46,18 @@
 extern "C" {
 #endif
 
+#if !defined(RIOT_EPOCH) || DOXYGEN
+/**
+ * @brief Earliest year of the RTC
+ *
+ * 01.01.$RIOT_EPOCH will be the reset value of the RTC if supported.
+ *
+ * Internal RTC helper functions such as @see rtc_mktime and @see rtc_localtime
+ * will not work on dates earlier than that.
+ */
+#define RIOT_EPOCH (2020)
+#endif
+
 /**
  * @brief Signature for alarm Callback
  *

drivers/periph_common/rtc.c

@@ -31,10 +31,6 @@
 #define HOUR    (60U * MINUTE)
 #define DAY     (24U * HOUR)
 
-#ifndef RIOT_EPOCH
-#define RIOT_EPOCH (2020)
-#endif
-
 /*
  * The rules here are (to be checked in that explicit order):
  *  1. If the year is not a multiple of four, it is not a leap year.