This is the first tool in the CMake build, so some extra
noise is involved:

- Setup tools list vars and handling.
- Add tools support to the dist rule.
- Move usage_exit.c generation to CMakelists.txt to allow
  use by the aom_entropy_optimizer target.

BUG=aomedia:834

Change-Id: I55239e89353033349ac1038b8d3d1aa8a8f23e27