Consider making buildmanager $config['build']['build_file'] accept an array.

[bryanhirsch]

I'm beginning to think people should be able to do this:

build:
  build_file: build.make

and this:

build:
  build_file:
    - build.make
    - build-dev.make

User story: As a developer, I want to be able to do dev builds that point all the projects I maintain at the tip of the dev branches. Maintaining separate builmanager config files for this is a headache because it's easy for the files to get out of sync. I should be able to provide a list of possible make files, then drush should prompt me to select which I want to use when Build Manager discovers more than one option.