Allow buildrequiring modules built against all platform streams.
This commit fixes issue with following situation:
Module `foo` has been built with `buildrequires: platform: [f28]`
and `requires: platform: []`. It can therefore be used as a buildrequirement
on any platform stream. But if you want to build module `app` which
buildrequires `foo` on `platform:f30`, the MBS won't pull-in the `foo`
module build, because MBS currently limits the modules by the platform
they have been built for.
This commit adds new config option to allow including modules built
against any platform stream.