Add an rtc module that provides a singleton RTC class with
- a datetime property to set and get time if the board supports it.
- a calbration property to adjust the clock.
There's also an rtc.set_time_source() method to override this RTC object using pure python.
The time module gets 3 methods:
- time.time()
- time.localtime()
- time.mktime()
The rtc timesource is used to provide time to the time module.
lib/timeutils is used for time conversions and thus only supports dates after 2000.
