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>