Organize aom_config_defaults.cmake and update docstrings.
Organize aom_config_defaults.cmake and update docstrings.

Change-Id: Ibf73c7fa69adfe83f6d57546b7a8f9da67638048

GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom