autoboot: Rename CONFIG_MENU_SHOW to include AUTOBOOT

Rename this option to CONFIG_AUTOBOOT_MENU_SHOW this it relates to the
autoboot functionality.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/doc/README.bootmenu b/doc/README.bootmenu
index 34ff8d5..ca50990 100644
--- a/doc/README.bootmenu
+++ b/doc/README.bootmenu
@@ -91,7 +91,7 @@
 
   #define CONFIG_AUTOBOOT_KEYED
   #define CONFIG_BOOTDELAY 30
-  #define CONFIG_MENU_SHOW
+  #define CONFIG_AUTOBOOT_MENU_SHOW
 
 When you intend to use the bootmenu on color frame buffer console,
 make sure to additionally define CONFIG_CFB_CONSOLE_ANSI in the