eficonfig: refactor change boot order implementation

All the eficonfig menus other than "Change Boot Order"
use 'eficonfig_entry' structure for each menu entry.
This commit refactors change boot order implementation
to use 'eficonfig_entry' structure same as other menus
to have consistent menu handling.

This commit also simplifies the data->active handling when
KEY_SPACE is pressed, and sizeof() parameter.

Signed-off-by: Masahisa Kojima <masahisa.kojima@linaro.org>
Acked-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>