doc: flatten doxygen-generated HTML structure

The current doxygen configuration settings create 4096 folders to hold
the generated HTML output (with names such as d3/ddb/).  This makes it
difficult to know what an URL would be for a particular module's
(defgroup's) documentation.  By not creating these subdirectories,
module documentation file names become quite predictable:

     group__<group name>.html

(e.g., group__sensor__interface.html) allowing us to create
links from the Sphinx-generated documentation into the doxygen-generated
HTML content (this part is under investigation).

Signed-off-by: David B. Kinder <david.b.kinder@intel.com>