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/src/esp_timer.c

@@ -97,7 +97,6 @@ static StaticQueue_t s_timer_semaphore_memory;
 static portMUX_TYPE s_timer_lock = portMUX_INITIALIZER_UNLOCKED;
 
 
-
 esp_err_t esp_timer_create(const esp_timer_create_args_t* args,
                            esp_timer_handle_t* out_handle)
 {
@@ -349,7 +348,6 @@ static IRAM_ATTR bool is_initialized(void)
     return s_timer_task != NULL;
 }
 
-
 esp_err_t esp_timer_init(void)
 {
     esp_err_t err;
@@ -380,6 +378,11 @@ esp_err_t esp_timer_init(void)
         goto out;
     }
 
+    err = esp_timer_timekeeping_impl_init();
+    if (err != ESP_OK) {
+        goto out;
+    }
+
     return ESP_OK;
 
 out:
@@ -505,4 +508,4 @@ int64_t IRAM_ATTR esp_timer_get_next_alarm(void)
     }
     timer_list_unlock();
     return next_alarm;
-}
+}