autoboot: Rename CONFIG_MENUKEY to CONFIG_AUTOBOOT_MENUKEY

Since this is part of the autoboot functionality, it makes sense to name
it with an AUTOBOOT prefix. No mainline boards use it so this should be
safe, and downstream boards will need to adjust.

Since this option is just an integer value, it really needs another option
to control whether the feature is enabled or not. Add a new
CONFIG_USE_AUTOBOOT_MENUKEY for that. This fits better with how things are
done with Kconfig, avoiding the need to use a specific value to disable
the feature.

Signed-off-by: Simon Glass <sjg@chromium.org>