add some debuggers to help figure out why nfc-list doesn't see libusb devices

src/lib/drivers/acr122.c

@@ -144,7 +144,11 @@ acr122_list_devices(nfc_device_desc_t pnddDevices[], size_t szDevices, size_t *p
   *pszDeviceFound = 0;
 
   // Test if context succeeded
-  if (!(pscc = acr122_get_scardcontext ())) return false;
+  if (!(pscc = acr122_get_scardcontext ()))
+  {
+    DBG("%s","PCSC-LITE daemon not found");
+    return false;
+  }
 
   // Retrieve the string array of all available pcsc readers
   if (SCardListReaders(*pscc,NULL,acDeviceNames,(void*)&szDeviceNamesLen) != SCARD_S_SUCCESS) return false;

src/lib/drivers/pn531_usb.c

@@ -100,6 +100,7 @@ nfc_device_t* pn531_usb_connect(const nfc_device_desc_t* pndd)
   us.uiEndPointOut = 0;
   us.pudh = NULL;
 
+  DBG("%s", "Looking for PN531 device");
   usb_init();
   if (usb_find_busses() < 0) return NULL;
   if (usb_find_devices() < 0) return NULL;

src/lib/drivers/pn533_usb.c

@@ -98,6 +98,7 @@ nfc_device_t* pn533_usb_connect(const nfc_device_desc_t* pndd)
   us.uiEndPointOut = 0;
   us.pudh = NULL;
 
+  DBG("%s", "Looking for PN533 device");
   usb_init();
   if (usb_find_busses() < 0) return NULL;
   if (usb_find_devices() < 0) return NULL;

src/lib/nfc.c

@@ -94,12 +94,17 @@ nfc_list_devices(nfc_device_desc_t pnddDevices[], size_t szDevices, size_t *pszD
   {
     if (drivers_callbacks_list[uiDriver].list_devices != NULL)
     {
+      DBG("Checking driver: %s",drivers_callbacks_list[uiDriver]);
       size_t szN = 0;
       if (drivers_callbacks_list[uiDriver].list_devices (pnddDevices + (*pszDeviceFound), szDevices - (*pszDeviceFound), &szN))
       {
         *pszDeviceFound += szN;
       }
     }
+    #ifdef DEBUG
+    else
+      DBG("Not checking driver: %s",drivers_callbacks_list[uiDriver]);
+    #endif
   }
 }
 
@@ -115,6 +120,7 @@ nfc_device_t* nfc_connect(nfc_device_desc_t* pndd)
   {
     if (pndd == NULL) {
       // No device description specified: try to automatically claim a device
+      DBG("%s","Autodetecting available devices...");
       pndd = drivers_callbacks_list[uiDriver].pick_device ();
       pnd = drivers_callbacks_list[uiDriver].connect(pndd);
     } else {