Add aom_entropy_optimizer to CMake build.
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