resourceloader: Move 'mw.config' assignment from startup.js to mediawiki.js

resourceloader: Move 'mw.config' assignment from startup.js to mediawiki.js

Before this change, the startup module responds as follows:

   ```lang=js
   ... contents of startup/mediawiki.js:
   ...... mw.config = null;
   ...
   ... contents of startup/startup.js:
   ...... mw.config = new mw.Map( bool );
   ```

After this change:

   ```lang=js
   ... contents of startup/mediawiki.js:
   ...... mw.config = new mw.Map( bool );
   ...
   ... contents of startup/startup.js:
   ......
   ```

Change-Id: I97fea20f17c4865aa4740482f3054532038531f0