add serial interface and a few std library functions

changes in usart:
  - new functions to setup the serial comunication settings such as baudrate,
    parity and stop bits
  - init function with most common values
  - transmit and receive functions each with a wrapper to send data blocks

changes in libc:
  - new file stdio.c with basic implementation of putch, printf still a prototype
  - new file string.c with memcpy() function

1 files changed, 11 insertions, 0 deletions

diff --git a/sw/z80/libc/include/stdio.h b/sw/z80/libc/include/stdio.h
new file mode 100644
index 0000000..b31cdfd
--- /dev/null
+++ b/sw/z80/libc/include/stdio.h
@@ -0,0 +1,11 @@
+#ifndef __STDIO_H__
+#define __STDIO_H__
+
+#include "types.h"
+
+extern uint8_t *stdout, stderr;
+
+void putc(char ch, uint8_t *buffer);
+int printf(const char *fmt, ...);
+
+#endif