update cdc_msc/cdc_msc_freertos to also support notification

examples/device/cdc_msc/src/main.c

@@ -119,6 +119,16 @@ void cdc_task(void) {
       tud_cdc_write(buf, count);
       tud_cdc_write_flush();
     }
+
+    // Press on-board button to send Uart status notification
+    static uint32_t btn_prev = 0;
+    static cdc_notify_uart_state_t uart_state = { .value = 0 };
+    const uint32_t btn = board_button_read();
+    if (!btn_prev && btn) {
+      uart_state.dsr ^= 1;
+      tud_cdc_notify_uart_state(&uart_state);
+    }
+    btn_prev = btn;
   }
 }