esp_system: introduce system time functions

- Introduce system time function and concept of system time provider.
esp_timer is system time provider when present.
- Set the reference point for system time, g_startup_time.
- Use the system time functions in newlib instead of calling esp_timer
functions directly

components/esp_timer/private_include/esp_timer_impl.h

@@ -117,3 +117,8 @@ uint64_t esp_timer_impl_get_counter_reg(void);
  * @return the value of the alarm register
  */
 uint64_t esp_timer_impl_get_alarm_reg(void);
+
+/**
+ * @brief Initialize and provide system timekeeping functions.
+ */
+esp_err_t esp_timer_timekeeping_impl_init(void);