app: aboot: update battery voltage value timely for getvar command

The buf which store the battery voltage is not a latest value. So
update the value before read it.

Change-Id: If2f233786d4ddcafeeba144c3d75c80c9d9f76bd
diff --git a/include/target.h b/include/target.h
index 6f83b6f..1778dd2 100644
--- a/include/target.h
+++ b/include/target.h
@@ -92,6 +92,10 @@
 uint32_t get_vibration_type();
 #endif
 
+#if CHECK_BAT_VOLTAGE
+void update_battery_status(void);
+#endif
+
 uint32_t target_get_battery_voltage();
 bool target_battery_soc_ok();
 bool target_battery_is_present();