Convert CONFIG_AUTOBOOT_MENU_SHOW to Kconfig

This converts the following to Kconfig:
   CONFIG_AUTOBOOT_MENU_SHOW
Signed-off-by: NSimon Glass <sjg@chromium.org>

cmd/Kconfig

@@ -170,6 +170,14 @@ config AUTOBOOT_MENUKEY
 	  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

configs/nokia_rx51_defconfig

@@ -12,6 +12,7 @@ CONFIG_SYS_CONSOLE_IS_IN_ENV=y
 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

include/configs/nokia_rx51.h

@@ -270,8 +270,6 @@ int rx51_kp_getc(struct stdio_dev *sdev);
 	"run attachboot;" \
 	"echo"
 
-#define CONFIG_AUTOBOOT_MENU_SHOW
-
 /*
  * Miscellaneous configurable options
  */

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