Add target_replace option in input_files

In tor-browser-build/projects/geckoview/config we currently use target_prepend to add targets like torbrowser-android-armv7. However we end up with having two different torbrowser-android-* targets at the same time. What we really want is to replace the current torbrowser-android-* target with a new one, so having an option to do that would be useful.