openblas: updates and other maintenance

This updates openblas to version 0.2.11, but retains versions 0.2.2 and 0.2.10, as julia is pinned to those versions. The `openblas` attribute points to the latest, though. I also added the config option `openblas.preferLocalBuild`, so those of us who prefer a locally-optimized build can get that instead. If you want to override the autodetected target, you can set `openblas.target`. Finally, I added myself as maintainer.