Only produce documentation where documentation actually exists.

src/doxygen.cfg

@@ -7,7 +7,7 @@ PROJECT_NAME           = Etherboot
 PROJECT_NUMBER         = 
 OUTPUT_DIRECTORY       = @BIN@/doc
 OUTPUT_LANGUAGE        = English
-EXTRACT_ALL            = YES
+EXTRACT_ALL            = NO
 EXTRACT_PRIVATE        = YES
 EXTRACT_STATIC         = YES
 EXTRACT_LOCAL_CLASSES  = YES