Modify add_xcode_base_configuration to respect existing base configurations

 - Do not change the base configuration when it is already set
 - Notify the user when there is an existing base configuration
The instructions should be more clear and should be a warning (`UI.warn`):

```
CocoaPods did not set the base configuration of your project because one is already set. Please either set the base configurations of the target `#{native_target.name}` to `#{xcconfig_path}` or include the `#{xcconfig_path}` in your build configuration.
```