Review Request: 8005850: build-infra: Make --enable-openjdk-only really disable custom

[Tim Bell]

Erik:

> The open part of this review. Move handling of --enable-openjdk-only 
> to before the first custom hook call.
>
> http://cr.openjdk.java.net/~erikj/8005850/webrev.root.01/

Looks good.

Tim