Feat: lab 3 w/o adv 2

bootloader/include/uart.h

@@ -0,0 +1,12 @@
+#pragma once
+
+#include <stddef.h>
+
+void uart_init();
+void uart_send(uint32_t c);
+uint8_t uart_getb();
+char uart_getc();
+void uart_puts(const char *s);
+void uart_hex(uint64_t d);
+
+extern int is_uart_inited;