Add DFU runtime and mode "class"

With the runtime and mode portions in separate classes, a single
application should only be building with one or the other enabled.  In
some applications both might be desired at build time.

The CFG_TUD_DFU_RUNTIME_AND_MODE option creates a DFU class, which asks
the application which mode to initialize to.  This allows a runtime
change between RT and DFU mode, by just reinitializing tusb.

src/device/usbd.c

@@ -199,6 +199,18 @@ static usbd_class_driver_t const _usbd_driver[] =
   },
   #endif
 
+  #if CFG_TUD_DFU_RUNTIME_AND_MODE
+  {
+    DRIVER_NAME("DFU-RT-MODE")
+    .init             = dfu_init,
+    .reset            = dfu_reset,
+    .open             = dfu_open,
+    .control_xfer_cb  = dfu_control_xfer_cb,
+    .xfer_cb          = NULL,
+    .sof              = NULL
+  },
+  #endif
+
   #if CFG_TUD_NET
   {
     DRIVER_NAME("NET")