backuptool: /system/addon.d explicit creation

 * Add the addon.d folder creation to preserve_addon_d
    before copying the files back to /system/addon.d

 * On CM based ROMs, the path exists for built elements,
    but on other AOSP-based ROMs where this script might
    be used, the folder might not be created on build and
    copy fails, breaking backuptool on second ROM update

Change-Id: I7438823b8d7ad0972649d2bf491d0f6fe423bc99