Publish `bbl` as Nix package

[geigerj0]

Problem

bbl is not available on https://search.nixos.org/packages. This means that no one using Nix development shells can easily provide this CLI.

Current workaround to still get bbl is to build it locally which is cumbersome, for example like so cloudfoundry/app-autoscaler-release#2594.

Feature Request

As a developer,
I want to find bbl-CLI tool in the Nix package registry https://search.nixos.org/packages,
so that I can offer a Nix shell with this tool to other contributors (example where I'd like to make use of it).

[rkoster]

Feel free to take: https://github.com/rkoster/nixpkgs/blob/master/pkgs/bosh-bootloader/default.nix and contribute it back to https://search.nixos.org/packages.