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>

common/cli_hush.c

@@ -79,6 +79,7 @@
 #include <malloc.h>         /* malloc, free, realloc*/
 #include <linux/ctype.h>    /* isalpha, isdigit */
 #include <common.h>        /* readline */
+#include <bootretry.h>
 #include <cli.h>
 #include <cli_hush.h>
 #include <command.h>        /* find_cmd */