Battery status: Add valid flag

msg/battery_status.msg

@@ -5,6 +5,7 @@ float32 current_filtered_a	# Battery current in amperes, filtered, 0 if unknown
 float32 discharged_mah		# Discharged amount in mAh, -1 if unknown
 float32 remaining		# From 1 to 0, -1 if unknown
 int32 cell_count		# Number of cells
+bool connected			# Wether or not a battery is connected
 #bool is_powering_off		# Power off event imminent indication, false if unknown