doxygen: switch to svg graphics
doxygen: switch to svg graphics

With this change doxygen will create SVG graphics (instead of png) which
are zoomable due these are vector graphics.

Suggested-by: Bill Bennet <842c0c5d3861399d031a2eaafd821d68e89f2d50@netcommwireless.com>

authmillenon/libcoap,gebart/libcoap,gebart/libcoap,authmillenon/libcoap,gebart/libcoap,authmillenon/libcoap