esp8266/modpybrtc: Handle RTC overflow.

ESP-SDK system_get_rtc_time() returns uint32 and therefore overflow
about every 7:45h.  Let's write the last state of system_get_rtc_time()
in RTC mem and use it to check for overflow. This commit require running
pyb_rtc_get_us_since_2000() at least once within 7 hours to avoid
overflow.