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