Allow setting target, target_replace, target_prepend in projects config

Currently to use the target, target_replace or target_prepend options, they need to be defined in input_files.

For some projects where we always want to change targets, it could be useful to be able to define those options directly in the project's config to avoid the need to redifine them in all the places where the project is used in input_files.

/cc @richard