Allowing urlPrefix override:

* Actually attaching buildOverridesDefinition tot he processed stuff so it's conctest can actually have effect
* assignPrefix should now not override urlPrefix if we catually have one assigned!
Nice catch! However, this line could be kept intact and as well as the other changed line, just the if block contents need be changed:

```javascript
buildOverrides = JSON.parse(buildOverridesDefinition);
```