binary addons: add platforms.txt to be able to control on which platforms an addon can be built

I realized that there might be binary addons in the future which cannot be built on all platforms so we need a way to specify on which platform every addon can/should be built. I first thought I'd go with platform-specific sub-directories (like we do with the addon dependency stuff) but then I realized that there might be addons that work on all but one platform. With platform-specific subdirectories we'd have to duplicate the same addon file in  multiple sub-directories and also remember to keep them all synced.

That's why I came up with the platforms.txt for every addon which can either contain "all" or a list of platforms for which an addon can be built. That way there's only one place where the addon is specified.