eficonfig: add "Edit Boot Option" menu entry

This commit adds the menu entry to edit the existing
BOOT#### variable contents.
User selects the item from the boot option list, then
user can edit the description, file path and optional_data.

Note that automatically generated boot option entry by bootmenu
to support the removable media device is filtered out and user
can not edit the automatically generated entry.

Signed-off-by: Masahisa Kojima <masahisa.kojima@linaro.org>