utils: build-kitkat smarter aosp invocation
utils: build-kitkat smarter aosp invocation

* Attempt to find the aosp binary with the following priority:
  1. If AOSP_BIN is set, use that
  2. If aosp is found in the shell $PATH
  3. Grab it from the web

mmaret-geny/docker-aosp,kylemanna/docker-aosp