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_common/include/esp_private/system_internal.h

@@ -61,6 +61,16 @@ void esp_reset_reason_set_hint(esp_reset_reason_t hint);
  */
 esp_reset_reason_t esp_reset_reason_get_hint(void);
 
+/** 
+ * @brief Set function which provides `esp_system_get_time`.
+ *
+ * @param time_fn function which provides system time
+ * @param resolution resolution in microseconds of the time provider function specified
+ */
+void esp_system_set_time_provider(esp_system_time_fn_t time_fn, uint32_t resolution);
+
+
+
 #ifdef __cplusplus
 }
 #endif