all: Rename absolute time-based functions to include "epoch".

For time-based functions that work with absolute time there is the need for
an Epoch, to set the zero-point at which the absolute time starts counting.
Such functions include time.time() and filesystem stat return values.  And
different ports may use a different Epoch.

To make it clearer what functions use the Epoch (whatever it may be), and
make the ports more consistent with their use of the Epoch, this commit
renames all Epoch related functions to include the word "epoch" in their
name (and remove references to "2000").

Along with this rename, the following things have changed:

- mp_hal_time_ns() is now specified to return the number of nanoseconds
  since the Epoch, rather than since 1970 (but since this is an internal
  function it doesn't change anything for the user).

- littlefs timestamps on the esp8266 have been fixed (they were previously
  off by 30 years in nanoseconds).

Otherwise, there is no functional change made by this commit.

Signed-off-by: Damien George <damien@micropython.org>

ports/esp8266/modmachine.h

@@ -32,8 +32,8 @@ void pin_set(uint pin, int value);
 extern uint32_t pyb_rtc_alarm0_wake;
 extern uint64_t pyb_rtc_alarm0_expiry;
 
-void pyb_rtc_set_us_since_2000(uint64_t nowus);
-uint64_t pyb_rtc_get_us_since_2000();
+void pyb_rtc_set_us_since_epoch(uint64_t nowus);
+uint64_t pyb_rtc_get_us_since_epoch();
 void rtc_prepare_deepsleep(uint64_t sleep_us);
 
 #endif // MICROPY_INCLUDED_ESP8266_MODMACHINE_H