battery: check source param inside battery lib

This moves the handling of the BAT%d_SOURCE param inside of the battery
library. Users of the library now pass the source instead of the flag
whether to publish. The battery library then checks if the source is
selected using the param and publishes accordingly.

Since we removed the strange system_source flag, we now need to look at
all batteries in commander.
For current estimation - I think - it makes sense to sum them up.

src/lib/battery/battery.cpp

@@ -1,6 +1,6 @@
 /****************************************************************************
  *
- *   Copyright (c) 2019 PX4 Development Team. All rights reserved.
+ *   Copyright (c) 2019-2020 PX4 Development Team. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
@@ -121,8 +121,8 @@ Battery::reset()
 
 void
 Battery::updateBatteryStatus(hrt_abstime timestamp, float voltage_v, float current_a,
-			     bool connected, bool selected_source, int priority,
-			     float throttle_normalized, bool should_publish)
+			     bool connected, int source, int priority,
+			     float throttle_normalized)
 {
 	reset();
 	_battery_status.timestamp = timestamp;
@@ -148,7 +148,7 @@ Battery::updateBatteryStatus(hrt_abstime timestamp, float voltage_v, float curre
 		_battery_status.warning = _warning;
 		_battery_status.remaining = _remaining;
 		_battery_status.connected = connected;
-		_battery_status.system_source = selected_source;
+		_battery_status.source = source;
 		_battery_status.priority = priority;
 
 		static constexpr int uorb_max_cells = sizeof(_battery_status.voltage_cell_v) / sizeof(
@@ -162,6 +162,8 @@ Battery::updateBatteryStatus(hrt_abstime timestamp, float voltage_v, float curre
 
 	_battery_status.timestamp = timestamp;
 
+	const bool should_publish = (source == _params.source);
+
 	if (should_publish) {
 		publish();
 	}