doc/doxygen: enable svg output
doc/doxygen: enable svg output

changed DOT_IMAGE_FORMAT to svg to get searchable dependency and call graphs
basilfx/RIOT,kaspar030/RIOT,basilfx/RIOT,authmillenon/RIOT,miri64/RIOT,OTAkeys/RIOT,ant9000/RIOT,authmillenon/RIOT,RIOT-OS/RIOT,authmillenon/RIOT,authmillenon/RIOT,authmillenon/RIOT,kYc0o/RIOT,miri64/RIOT,RIOT-OS/RIOT,miri64/RIOT,RIOT-OS/RIOT,jasonatran/RIOT,miri64/RIOT,kYc0o/RIOT,OTAkeys/RIOT,RIOT-OS/RIOT,basilfx/RIOT,OTAkeys/RIOT,OTAkeys/RIOT,OlegHahm/RIOT,kaspar030/RIOT,miri64/RIOT,ant9000/RIOT,kYc0o/RIOT,basilfx/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,authmillenon/RIOT,ant9000/RIOT,OlegHahm/RIOT,ant9000/RIOT,basilfx/RIOT,kYc0o/RIOT,kaspar030/RIOT,OlegHahm/RIOT,OlegHahm/RIOT,kYc0o/RIOT,OTAkeys/RIOT,ant9000/RIOT,jasonatran/RIOT,OlegHahm/RIOT,jasonatran/RIOT,jasonatran/RIOT,kaspar030/RIOT