Ability to push environment variables back into the build environment

[FISHMANPET]

There's currently a private function that will push the BH* variables back into your build environment. I have need to set a variable that's readable by my environment, and I was about to suggest that function be made public, then I realized it might make more sense to make a generic function that lets you set a variable and BH will figure out how to do it based on what build system you're in