menu: add KEY_PLUS, KEY_MINUS and KEY_SPACE handling

This is preparation to support menu-driven UEFI BootOrder
variable updated by KEY_PLUS, KEY_MINUS and KEY_SPACE.

Signed-off-by: Masahisa Kojima <masahisa.kojima@linaro.org>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>