esp_clk_rtc_time() function

Read value of RTC counter, converting it to microseconds @attention The value returned by this function may change abruptly when calibration value of RTC counter is updated via esp_clk_slowclk_cal_set function. This should not happen unless application calls esp_clk_slowclk_cal_set. In ESP-IDF, esp_clk_slowclk_cal_set is only called in startup code.