Doxygen interprets each Markdown input file as a separate section
(chapter). Concatenate all of the .md files in directories into a
single file per section to get a correctly-nested table of contents.
In particular, this matters for the navigation in the PDF output.
Change-Id: I778849d89da9a308136e43ac6cb630c4c2bbb3a5
Signed-off-by: Daniel Verkamp <daniel.verkamp@intel.com>
