Add battery_info message with serial number compatible with UAVCAN, MAVLink and drivers

I'm starting the separate battery info message because no char[32] should be published and logged
at high rate and we need a separate battery info message for static information as discussed.

msg/BatteryInfo.msg

@@ -0,0 +1,9 @@
+# Battery information
+#
+# Static or near-invariant battery information.
+# Should be streamed at low rate.
+
+uint64 timestamp # [us] Time since system start
+
+uint8 id # Must match the id in the battery_status message for the same battery
+char[32] serial_number # [@invalid 0 All bytes] Serial number of the battery pack in ASCII characters, 0 terminated

msg/CMakeLists.txt

@@ -47,6 +47,7 @@ set(msg_files
 	Airspeed.msg
 	AirspeedWind.msg
 	AutotuneAttitudeControlStatus.msg
+	BatteryInfo.msg
 	ButtonEvent.msg
 	CameraCapture.msg
 	CameraStatus.msg