autoboot: add necessary dependency at AUTOBOOT_MENU_SHOW

Otherwise, menu_show() will be undefined in bootdelay_process().

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
diff --git a/cmd/Kconfig b/cmd/Kconfig
index 98647f5..c61c851 100644
--- a/cmd/Kconfig
+++ b/cmd/Kconfig
@@ -172,6 +172,7 @@
 
 config AUTOBOOT_MENU_SHOW
 	bool "Show a menu on boot"
+	depends on CMD_BOOTMENU
 	help
 	  This enables the boot menu, controlled by environment variables
 	  defined by the board. The menu starts after running the 'preboot'