Enable with ./tools/configure.sh -l esp32-devkitc:dac
DAC channel 0 = GPIO 25
DAC channel 1 = GPIO 26
default path: /dev/dac0
Resolution 8 bits = values 0~255
Voltage: 0~Vref
The reference voltage 'Vref' here is input from the pin VDD3P3_RTC
which ideally equals to the power supply VDD (3.3V).