Convert CONFIG_AUTOBOOT_MENU_SHOW to Kconfig

This converts the following to Kconfig:
   CONFIG_AUTOBOOT_MENU_SHOW

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/nokia_rx51_defconfig b/configs/nokia_rx51_defconfig
index 5cd9601..ab5bb2d 100644
--- a/configs/nokia_rx51_defconfig
+++ b/configs/nokia_rx51_defconfig
@@ -12,6 +12,7 @@
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_PROMPT="Nokia RX-51 # "
 CONFIG_AUTOBOOT_KEYED=y
+CONFIG_AUTOBOOT_MENU_SHOW=y
 CONFIG_CMD_BOOTMENU=y
 # CONFIG_CMD_IMI is not set
 # CONFIG_CMD_SAVEENV is not set