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/cmd/Kconfig b/cmd/Kconfig
index 8d2e0a9..22a3737 100644
--- a/cmd/Kconfig
+++ b/cmd/Kconfig
@@ -170,6 +170,14 @@
 	  For example, 33 means "!" in ASCII, so pressing ! at boot would take
 	  this action.
 
+config AUTOBOOT_MENU_SHOW
+	bool "Show a menu on boot"
+	help
+	  This enables the boot menu, controlled by environment variables
+	  defined by the board. The menu starts after running the 'preboot'
+	  environmnent variable (if enabled) and before handling the boot delay.
+	  See README.bootmenu for more details.
+
 endmenu
 
 config BUILD_BIN2C
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
diff --git a/include/configs/nokia_rx51.h b/include/configs/nokia_rx51.h
index b251061..fd755bb 100644
--- a/include/configs/nokia_rx51.h
+++ b/include/configs/nokia_rx51.h
@@ -270,8 +270,6 @@
 	"run attachboot;" \
 	"echo"
 
-#define CONFIG_AUTOBOOT_MENU_SHOW
-
 /*
  * Miscellaneous configurable options
  */
diff --git a/include/configs/vexpress_aemv8a.h b/include/configs/vexpress_aemv8a.h
index 2b1269d..89cd8b5 100644
--- a/include/configs/vexpress_aemv8a.h
+++ b/include/configs/vexpress_aemv8a.h
@@ -110,8 +110,6 @@
 #define CONFIG_PL011_CLOCK		24000000
 #endif
 
-/*#define CONFIG_AUTOBOOT_MENU_SHOW*/
-
 /* BOOTP options */
 #define CONFIG_BOOTP_BOOTFILESIZE