AP_BattMonitor: make C_TO_KELVIN a function macro; create KELVIN_TO_C

These are in celsius

libraries/AP_BattMonitor/AP_BattMonitor_SMBus.cpp

@@ -117,7 +117,7 @@ void AP_BattMonitor_SMBus::read_temp(void)
     _has_temperature = true;
 
     _state.temperature_time = AP_HAL::millis();
-    _state.temperature = (data * 0.1f) - C_TO_KELVIN;
+    _state.temperature = KELVIN_TO_C(0.1f * data);
 }
 
 // reads the serial number if it's not already known

libraries/AP_BattMonitor/AP_BattMonitor_SMBus_Rotoye.cpp

@@ -17,6 +17,6 @@ void AP_BattMonitor_SMBus_Rotoye::read_temp(void) {
         uint16_t t;
         _state.temperature_time = AP_HAL::millis();
         t = ((t_ext > t_int) ? t_ext : t_int);
-        _state.temperature = 0.1f * (float)t - C_TO_KELVIN;
+        _state.temperature = KELVIN_TO_C(0.1f * (float)t);
     }
 }

libraries/AP_BattMonitor/AP_BattMonitor_UAVCAN.cpp

@@ -130,7 +130,7 @@ void AP_BattMonitor_UAVCAN::update_interim_state(const float voltage, const floa
 
     if (!isnanf(temperature_K) && temperature_K > 0) {
         // Temperature reported from battery in kelvin and stored internally in Celsius.
-        _interim_state.temperature = temperature_K - C_TO_KELVIN;
+        _interim_state.temperature = KELVIN_TO_C(temperature_K);
         _interim_state.temperature_time = AP_HAL::millis();
     }
 
@@ -185,7 +185,7 @@ void AP_BattMonitor_UAVCAN::handle_mppt_stream(const MpptStreamCb &cb)
     const uint8_t soc = 127;
 
     // convert C to Kelvin
-    const float temperature_K = isnanf(cb.msg->temperature) ? 0 : cb.msg->temperature + C_TO_KELVIN;
+    const float temperature_K = isnanf(cb.msg->temperature) ? 0 : C_TO_KELVIN(cb.msg->temperature);
 
     update_interim_state(voltage, current, temperature_K, soc);