bootmenu: U-Boot console is enabled as default

The commit 2158b0da ("bootmenu: add Kconfig option
not to enter U-Boot console") disables to enter U-Boot
console from bootmenu as default, this change affects the
existing bootmenu users.

This commit reverts the default behavior, the bootmenu can
enter U-Boot console same as before.
CMD_BOOTMENU_ENTER_UBOOT_CONSOLE is renamed
BOOTMENU_DISABLE_UBOOT_CONSOLE and depends on
AUTOBOOT_MENU_SHOW.

Fixes: 2158b0da ("bootmenu: add Kconfig option not to enter U-Boot console")
Signed-off-by: Masahisa Kojima <masahisa.kojima@linaro.org>
Tested-by: Pali Rohar <pali@kernel.org>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>