Configuration API

Setting up various configurations in buck2 has a lot of associated vocabulary which is non-clear.

"platform" and "configuration" mean more or less the same thing, from the documentation:

> buck.target_configuration and buck.execution_platform attributes which tell you what the target is being built for and what it's being built on

Calling them `buck.target_configuration` and `buck.execution_configuration` would be much clearer. I believe at that point it would be possible to drop the platform name all together and have a `configuration` rule. (A soft migration could be done using the feature discussed in #456 )