Move bootretry code into bootretry.c and clean up

This code is only used by one board, so it seems a shame to clutter up
the readline code with it. Move it into its own file.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/board/hymod/input.c b/board/hymod/input.c
index 59ad6aa..2f857c0 100644
--- a/board/hymod/input.c
+++ b/board/hymod/input.c
@@ -6,6 +6,7 @@
  */
 
 #include <common.h>
+#include <bootretry.h>
 #include <cli.h>
 
 int