added vxworks console command for reading the battery voltage

Reading the voltage to avoid destroying the battery was the only reason to start
up the drivers station.
diff --git a/aos/crio/controls/JoyStickRead.cpp b/aos/crio/controls/JoyStickRead.cpp
index 043716b..1624c40 100644
--- a/aos/crio/controls/JoyStickRead.cpp
+++ b/aos/crio/controls/JoyStickRead.cpp
@@ -84,6 +84,13 @@
   }
 };
 
+// Designed to be called from the console.
+extern "C" int battery_voltage() {
+  printf("battery is currently at %fV\n",
+         ControlsManager::GetInstance().GetDS()->GetBatteryVoltage());
+  return 0;
+}
+
 }  // namespace crio
 }  // namespace aos