Battery voltage read implemented (TTGOv2.1)

src/hal/ttgov21.h

@@ -5,6 +5,7 @@
 #define HAS_DISPLAY U8X8_SSD1306_128X64_NONAME_HW_I2C
 #define DISPLAY_FLIP  1 // rotated display
 #define HAS_LED 23 // green on board LED_G3 (not in initial board version)
+#define HAS_BATTERY_PROBE ADC1_GPIO35_CHANNEL // battery probe GPIO pin -> ADC_CHANNEL_7
 
 // re-define pin definitions of pins_arduino.h
 #define PIN_SPI_SS    18 // ESP32 GPIO18 (Pin18) -- HPD13A NSS/SEL (Pin4) SPI Chip Select Input