doc: flatten doxygen-generated HTML structure
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 <c56c8b29908a5b2af1b63001268d73a0b7f972ba@intel.com>

zephyriot/zephyr,galak/zephyr,explora26/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,GiulianoFranchetto/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,mbolivar/zephyr,ldts/zephyr,Vudentz/zephyr,nashif/zephyr,punitvara/zephyr,nashif/zephyr,kraj/zephyr,galak/zephyr,punitvara/zephyr,kraj/zephyr,mbolivar/zephyr,Vudentz/zephyr,Vudentz/zephyr,zephyriot/zephyr,finikorg/zephyr,Vudentz/zephyr,finikorg/zephyr,GiulianoFranchetto/zephyr,explora26/zephyr,finikorg/zephyr,nashif/zephyr,explora26/zephyr,explora26/zephyr,ldts/zephyr,zephyrproject-rtos/zephyr,kraj/zephyr,galak/zephyr,kraj/zephyr,Vudentz/zephyr,GiulianoFranchetto/zephyr,zephyriot/zephyr,ldts/zephyr,mbolivar/zephyr,explora26/zephyr,galak/zephyr,Vudentz/zephyr,punitvara/zephyr,ldts/zephyr,GiulianoFranchetto/zephyr,GiulianoFranchetto/zephyr,zephyriot/zephyr,zephyriot/zephyr,kraj/zephyr,mbolivar/zephyr,punitvara/zephyr,punitvara/zephyr,finikorg/zephyr,ldts/zephyr,mbolivar/zephyr