Integrate Build Configs for iOS

[maxammann]

Is your feature request related to a problem? Please describe.
The goal of this task is to make every option from the build config available in the configuration interface of XCode.

Describe the solution you'd like
This is done by converting the build config to an .xcconfig file. The tooling for this can be copied from the integreat project.

• If possible make sure that we only have a single tool for managing the whitelabeling. Do not create separate tools for different tasks.
• It would be very nice to be able to select the build config from within XCode. In Integreat we do this by using different build schemes. This is not a good solution because XCode modifies files when the app is started through different schemes.
  Instead it may be good to configure the current build config though a plain configuration file. When an XCode build is triggered, then the build config is choosen based on this file

Describe alternatives you've considered
None

Additional context
None

[maxammann]

Done in #551