Better support for repeatable builds

[AlekSi]

Take a look at the Makefile in that PR: https://github.com/talos-systems/toolchain/pull/24/files It builds a target twice and compares results, pruning the whole cache between builds. I think it has two problems that should be addressed by bldr:

1. That functionality should be a part of the bldr itself, not in a Makefile.
2. Pruning the whole cache might be too much, but just passing --no-cache is not enough. Maybe bldr could be modified to ignore cache or prune just needed entries.