AP_Bootloader: check VBUS for fast boot

Tools/AP_Bootloader/AP_Bootloader.cpp

@@ -106,6 +106,14 @@ int main(void)
         try_boot = false;
         timeout = 0;
     }
+#endif
+#if defined(HAL_GPIO_PIN_VBUS) && defined(HAL_ENABLE_VBUS_CHECK)
+#if HAL_USE_SERIAL_USB == TRUE
+    else if (palReadLine(HAL_GPIO_PIN_VBUS) == 0)  {
+        try_boot = true;
+        timeout = 0;
+    }
+#endif
 #endif
 
     // if we fail to boot properly we want to pause in bootloader to give