execute after copy to platforms dir rather than before

I think it's bad practice for us to modify files in App_Resources at build time, it would be ideal if we could append or create build.xcconfig in the platforms/ios folder instead after preparing.

[spike1292]

good suggestion, but currently im having problems with hooks which add files the xcode project afterwards.
Im thinking about making a poc to demonstrate the problems I'm currently having.
But a pull request to implement your suggestion is welcome!