bootmenu: add Kconfig option not to enter U-Boot console

This commit adds the Kconfig option to disable to enter
the U-Boot console from bootmenu.

If CMD_BOOTMENU_ENTER_UBOOT_CONSOLE is enabled, "U-Boot console"
entry is appeared as the last entry in the bootmenu, then user can
enter U-Boot console.

If CMD_BOOTMENU_ENTER_UBOOT_CONSOLE is disabled, "Quit" entry
is appeared as the last entry instead of "U-Boot console".
When user chooses "Quit" from bootmenu, the following default
commands are invoked.

 - "bootefi bootmgr" (if efi bootmgr is enabled)
 - "run bootcmd"

If the both commands are executed and returns to the bootmenu,
the bootmenu will appears again.

Signed-off-by: Masahisa Kojima <masahisa.kojima@linaro.org>
diff --git a/cmd/Kconfig b/cmd/Kconfig
index 09193b6..69c1814 100644
--- a/cmd/Kconfig
+++ b/cmd/Kconfig
@@ -357,6 +357,16 @@
 	help
 	  Add an ANSI terminal boot menu command.
 
+config CMD_BOOTMENU_ENTER_UBOOT_CONSOLE
+	bool "Allow Bootmenu to enter the U-Boot console"
+	depends on CMD_BOOTMENU
+	default n
+	help
+	  Add an entry to enter U-Boot console in bootmenu.
+	  If this option is disabled, user can not enter
+	  the U-Boot console from bootmenu. It increases
+	  the system security.
+
 config CMD_ADTIMG
 	bool "adtimg"
 	help