AXP192 lib updates

include/payload.h

@@ -100,6 +100,6 @@ private:
 };
 
 extern PayloadConvert payload;
-extern uint8_t batt_level;
+extern int8_t batt_level;
 
 #endif // _PAYLOAD_H_

include/power.h

@@ -131,6 +131,6 @@ static inline uint8_t linear(uint16_t voltage, uint16_t minVoltage,
          (maxVoltage - minVoltage);
 }
 
-uint8_t read_battlevel(mapFn_t mapFunction = &sigmoidal);
+int8_t read_battlevel(mapFn_t mapFunction = &sigmoidal);
 
 #endif