rak3x72 : report bat voltage

variants/rak3x72/target.h

@@ -8,11 +8,19 @@
 #include <helpers/ArduinoHelpers.h>
 #include <helpers/SensorManager.h>
 
+#define  PIN_VBAT_READ    A0
+#define  ADC_MULTIPLIER   (5 * 1.73 * 1000)
+
 class RAK3x72Board : public STM32Board {
 public:
     const char* getManufacturerName() const override {
         return "RAK 3x72";
     }
+
+    uint16_t getBattMilliVolts() override {
+        uint32_t raw = analogRead(PIN_VBAT_READ);            
+        return (ADC_MULTIPLIER * raw) / 1024;
+    }
 };
 
 extern RAK3x72Board board;