sandbox: put stdin into raw mode

This allows us to act like a serial device: we get tab chars and CTRL+C
and respond appropriately.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Tested-by: Simon Glass <sjg@chromium.org>

include/os.h

@@ -71,3 +71,8 @@ int os_close(int fd);
  * @param exit_code	exit code for U-Boot
  */
 void os_exit(int exit_code);
+
+/**
+ * Put tty into raw mode to mimic serial console better
+ */
+void os_tty_raw(int fd);