commander: move battery calculations to systemlib

The commander used to consume the battery_status topic and write the
contents after some calculations into the system state. Instead, the
calculations now happen in library calls in systemlib/battery.

This moves the battery fields out of the vehicle_status message into the
battery_status topic.

This brought quite some changes in all modules that need battery
information. The current state is compiling but untested.

msg/battery_status.msg

@@ -3,4 +3,13 @@ float32 voltage_v		# Battery voltage in volts, 0 if unknown
 float32 voltage_filtered_v	# Battery voltage in volts, filtered, 0 if unknown
 float32 current_a		# Battery current in amperes, -1 if unknown
 float32 discharged_mah		# Discharged amount in mAh, -1 if unknown
-bool is_powering_off		# Power off event imminent indication, false if unknown
+float32 remaining		# From 1 to 0, -1 if unknown
+int32 cell_count		# Number of cells
+#bool is_powering_off		# Power off event imminent indication, false if unknown
+
+
+uint8 BATTERY_WARNING_NONE = 0       # no battery low voltage warning active
+uint8 BATTERY_WARNING_LOW = 1        # warning of low voltage
+uint8 BATTERY_WARNING_CRITICAL = 2   # alerting of critical voltage
+
+uint8 warning    # current battery warning

msg/vehicle_status.msg

@@ -77,7 +77,6 @@ bool is_vtol				# True if the system is VTOL capable
 bool vtol_fw_permanent_stab		# True if vtol should stabilize attitude for fw in manual mode
 bool in_transition_mode			# True if VTOL is doing a transition
 
-bool condition_battery_voltage_valid
 bool condition_system_in_air_restore		# true if we can restore in mid air
 bool condition_system_sensors_initialized
 bool condition_system_prearm_error_reported	# true if errors have already been reported
@@ -125,11 +124,4 @@ uint32 onboard_control_sensors_enabled
 uint32 onboard_control_sensors_health
 
 float32 load					# processor load from 0 to 1
-float32 battery_voltage
-float32 battery_current
-float32 battery_remaining
-float32 battery_discharged_mah
-uint32 battery_cell_count
-
-uint8 battery_warning    # current battery warning mode, as defined by VEHICLE_BATTERY_WARNING enum