doxygen support

Add a Doxyfile configuration for Doxygen.

Usage: run doxygen from doc directory to generate the LRC documentation.
The src/ directory is the source for inputs, scanned recursively.
A doc/html/ directory is then filler with all HTML files.
Browse index.html to read the LRC documentation.

Change-Id: Ieb4b5db6f690f47fab2e40358a142fe973412e9c
Reviewed-by: Nicolas Jäger <nicolas.jager@savoirfairelinux.com>