sandbox: Allow Ctrl-C to work in sandbox

It is useful for Cltl-C to be handled by U-Boot as it is on other boards.
But it is also useful to be able to terminate U-Boot with Ctrl-C.

Add an option to enable signals while in raw mode, and make this the
default. Add an option to leave the terminal cooked, which is useful for
redirecting output.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/include/os.h b/include/os.h
index 9b5da5c..0230a7f 100644
--- a/include/os.h
+++ b/include/os.h
@@ -103,8 +103,12 @@
 
 /**
  * Put tty into raw mode to mimic serial console better
+ *
+ * @param fd		File descriptor of stdin (normally 0)
+ * @param allow_sigs	Allow Ctrl-C, Ctrl-Z to generate signals rather than
+ *			be handled by U-Boot
  */
-void os_tty_raw(int fd);
+void os_tty_raw(int fd, bool allow_sigs);
 
 /**
  * Acquires some memory from the underlying os.