Include sleep into library

libc/library.c

@@ -1,6 +1,15 @@
 #include "library.h"
 #include "os/driver/usart.h"
+#include "os/delay.h"
 
 void puts(const char *s) {
     usart_tx_write_string(s);
 }
+
+void usleep(unsigned int usec) {
+    delay_ms(usec);
+}
+
+void sleep(unsigned int sec) {
+    usleep(sec*1000);
+}