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/common/cmd_mem.c b/common/cmd_mem.c
index 15a4b84..9bd7c0e 100644
--- a/common/cmd_mem.c
+++ b/common/cmd_mem.c
@@ -12,6 +12,7 @@
  */
 
 #include <common.h>
+#include <bootretry.h>
 #include <cli.h>
 #include <command.h>
 #ifdef CONFIG_HAS_DATAFLASH