[FIX] config: correctly bootstrap data_dir

Complements the patch in 4715d18e
in order to properly bootstrap a writeable data_dir when it is
(partially) nonexistant.
Depending on the startup parameters the data_dir might otherwise
have ended up read-only, preventing the creation of its necessary
components (session store, file store). Only the `addons` directory
of the data_dir needs to be read-only by default.