Changes in this commit:
- Start the RTC Timer at system boot. Otherwise time.time() will advance
only if an RTC() object was created.
- Set the time to a more recent date than Jan 1, 1970, if not set. That is
2013/10/14, 19:53:11, MicroPython's first commit.
- Compensate an underflow in in timeutils_seconds_since_2000(), called by
time.time(), if the time is set to a pre-2000 date.