Add support for building a specific platform

[laeubi]

Currently one can define multiple platforms in the target to build against. This currently allows building for multiple platforms but also has the drawback that possibly things are on the classpath that are not there on runtime.

As its today usual to build on different platforms in a matrix build, there should be an option to build for a specific platform, such a build should work as follows:

1. The resolving an building should behave as if there is exactly one platform defined (that is the current running platform)
2. Projects that have a platform filter that do not match should be skipped