multi-target builds

[mzihlmann]

We currently only build one target at a time. Technically there is nothing that prevents us from building multiple targets. This will bring a huge performance benefit for cases where we have a combinatoric explosion via build parameters (ie. toolchain versions). However, I'm not sure how to best expose multi-target builds to the user over the existing cli interface. A proper clean solution would be to support bakefiles. But bakefiles will be not-so-easy to integrate I guess, maybe it's possible to support simple multi-target builds prior to supporting entire bakefiles?