Add example USART driver

libc/library.c

@@ -1,10 +1,6 @@
 #include "library.h"
-
-volatile unsigned int *const USART1_PTR = (unsigned int *)0x40011004;
+#include "os/driver/usart.h"
 
 void puts(const char *s) {
-    while(*s != '\0') { /* Loop until end of string */
-        *USART1_PTR= (unsigned int)(*s); /* Transmit char */
-        s++; /* Next char */
-    }
+    usart_tx_write_string(s);
 }