start adding U[S]ART support (need DMA)

F3:F303/InterfaceBoard/usart.h

@@ -0,0 +1,26 @@
+/*
+ * This file is part of the multiiface project.
+ * Copyright 2026 Edward V. Emelianov <edward.emelianoff@gmail.com>.
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#pragma once
+
+#include "hardware.h"
+#include "usb_dev.h"
+
+void usarts_setup();
+void usart_config(uint8_t ifNo, usb_LineCoding *lc);
+void usart_sendn(uint8_t ifNo, const uint8_t *str, int L);