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, 13 insertions, 0 deletions

diff --git a/sw/z80/libc/string.c b/sw/z80/libc/string.c
new file mode 100644
index 0000000..fd6a7ff
--- /dev/null
+++ b/sw/z80/libc/string.c
@@ -0,0 +1,13 @@
+#include "string.h"
+
+void *memcpy(void *dest, void *src, size_t n)
+{
+    char *dp = dest;
+    char *sp = src;
+
+    while (n--) {
+        *dp++ = *sp++;
+    }
+
+    return dest;
+}