On the ESP32S2, rtc_clk_cal(RTC_CAL_RTC_MUX) measures the frequency
of the 90kHz RTC clock regardless of the selected slow clock
frequency. Keep track which clock is selected and pass the argument
to rtc_clk_cal accordingly.
fix clock choices
update rtc 32k xtal code for s2
missed api in rtc.h
bootloader_clock: update for S2
1. add brownout detector HAL for esp32 and esp32s2
2. enable brownout reset for esp32 rev. 1 and above
3. add approximate brownout detector levels for esp32s2
