HAL_PX4: fixed USB connected on AUAV-X2

this is the 2nd attempt at a fix for the usb_connected status on
AUAV-X2

libraries/AP_HAL_PX4/GPIO.cpp

@@ -245,10 +245,11 @@ bool PX4GPIO::usb_connected(void)
     struct stat st;
     /*
       we use a combination of voltage on the USB connector and the
-      existance of the /dev/ttyACM0 character device. This copes with
+      open of the /dev/ttyACM0 character device. This copes with
       systems where the VBUS may go high even with no USB connected
+      (such as AUAV-X2)
      */
-    return stm32_gpioread(GPIO_OTGFS_VBUS) && stat("/dev/ttyACM0", &st) == 0;
+    return stm32_gpioread(GPIO_OTGFS_VBUS) && _usb_connected;
 }
 
 

libraries/AP_HAL_PX4/GPIO.h

@@ -46,11 +46,15 @@ public:
     /* return true if USB cable is connected */
     bool usb_connected(void);
 
+    // used by UART code to avoid a hw bug in the AUAV-X2
+    void set_usb_connected(void) { _usb_connected = true; }
+
 private:
     int _led_fd;
     int _tone_alarm_fd;
     int _gpio_fmu_fd;
     int _gpio_io_fd;
+    bool _usb_connected = false;
 };
 
 class PX4::PX4DigitalSource : public AP_HAL::DigitalSource {

libraries/AP_HAL_PX4/UARTDriver.cpp

@@ -17,6 +17,7 @@
 #include <drivers/drv_hrt.h>
 #include <assert.h>
 #include "../AP_HAL/utility/RingBuffer.h"
+#include "GPIO.h"
 
 using namespace PX4;
 
@@ -146,6 +147,9 @@ void PX4UARTDriver::begin(uint32_t b, uint16_t rxS, uint16_t txS)
 
     if (_writebuf_size != 0 && _readbuf_size != 0 && _fd != -1) {
         if (!_initialised) {
+            if (strcmp(_devpath, "/dev/ttyACM0") == 0) {
+                ((PX4GPIO *)hal.gpio)->set_usb_connected();
+            }
             ::printf("initialised %s OK %u %u\n", _devpath, 
                      (unsigned)_writebuf_size, (unsigned)_readbuf_size);
         }