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/hymod.c b/board/hymod/hymod.c
index 55ffd67..f6990e9 100644
--- a/board/hymod/hymod.c
+++ b/board/hymod/hymod.c
@@ -8,6 +8,7 @@
  */
 
 #include <common.h>
+#include <bootretry.h>
 #include <cli.h>
 #include <mpc8260.h>
 #include <mpc8260_irq.h>