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>
diff --git a/doc/zephyr.doxyfile b/doc/zephyr.doxyfile
index 201ee1d..9579745 100644
--- a/doc/zephyr.doxyfile
+++ b/doc/zephyr.doxyfile
@@ -68,7 +68,7 @@
 # performance problems for the file system.
 # The default value is: NO.
 
-CREATE_SUBDIRS         = YES
+CREATE_SUBDIRS         = NO
 
 # The OUTPUT_LANGUAGE tag is used to specify the language in which all
 # documentation generated by doxygen is written. Doxygen will use this