esp_sleep_register_event_callback() function

- ESP_OK on success - ESP_ERR_INVALID_ARG if the input parameter event_cb_conf is NULL or event_id is out of range - ESP_ERR_NO_MEM if the remaining memory is insufficient to support malloc - ESP_FAIL if register the same function repeatedly

Some of these callback functions are called from IDLE task context hence they cannot call any blocking functions Passing NULL value will not deregister the callbacks, it will silently ignore and return ESP_OK