buildkit: Refactor multi-platform build process

Introduced the buildResult type that represents the result of a single image build. It contains a reference to the resulting image, the image configuration, extra build info, the target platform, platform export information, and whether the single result is part of a larger multi-platform build.

Factored out the call to CompileToLLB and the LLB solve (image build), and image config serialization into a buildImage function that returns a new buildResult. The buildResult can then be used to add/set the correct meta data and reference to the final aggregate result.

Follows up 43ec449d