doxygen: png -> gif for images

This reduces the amount of output in docs/ from 17G to 10G for me. No
big surprise, inheritance diagrams use only a few colors, so gif's
indexed colors work here nicely.

Thanks Christian Lohmaier for the idea.

Change-Id: I23db0c1a4dc7410cb3eb1ae99044962959fac28a

solenv/inc/doxygen.cfg

@@ -1832,7 +1832,7 @@ DIRECTORY_GRAPH        = YES
 # HTML_FILE_EXTENSION to xhtml in order to make the SVG files
 # visible in IE 9+ (other browsers do not have this requirement).
 
-DOT_IMAGE_FORMAT       = png
+DOT_IMAGE_FORMAT       = gif
 
 # If DOT_IMAGE_FORMAT is set to svg, then this option can be set to YES to
 # enable generation of interactive SVG images that allow zooming and panning.