seperate tusb_task() to tud_task() and tuh_task()

tusb_task() still exists for backward compatible

src/device/usbd.c

@@ -208,8 +208,24 @@ static void usbd_reset(uint8_t rhport)
 
 /* USB Device Driver task
  * This top level thread manages all device controller event and delegates events to class-specific drivers.
+ * This should be called periodically within the mainloop or rtos thread.
+ *
+   @code
+    int main(void)
+    {
+      application_init();
+      tusb_init();
+
+      while(1) // the mainloop
+      {
+        application_code();
+
+        tud_task(); // tinyusb device task
+      }
+    }
+    @endcode
  */
-void usbd_task (void)
+void tud_task (void)
 {
   // Loop until there is no more events in the queue
   while (1)

src/device/usbd.h

@@ -81,6 +81,7 @@ extern tud_desc_set_t tud_desc_set;
 // APPLICATION API
 //--------------------------------------------------------------------+
 bool tud_mounted(void);
+void tud_task (void);
 
 //--------------------------------------------------------------------+
 // APPLICATION CALLBACK (WEAK is optional)

src/device/usbd_pvt.h

@@ -52,8 +52,6 @@ extern tud_desc_set_t const* usbd_desc_set;
 // INTERNAL API for stack management
 //--------------------------------------------------------------------+
 bool usbd_init (void);
-void usbd_task (void);
-
 
 // Carry out Data and Status stage of control transfer
 // - If len = 0, it is equivalent to sending status only