Update DOXYGEN_EXCLUDE variable while make parses subfolders folders
Update DOXYGEN_EXCLUDE variable while make parses subfolders folders

Initially all subfolders of each Rules.mk file are added to this
variable. Then each Rules.mk file "filters-out" its own folder. This way
at the end this variable will have a list of all folders that were not
parsed, because they were disabled by configuration.
CezaryGapinski/distortos,DISTORTEC/distortos,CezaryGapinski/distortos,jasmin-j/distortos,DISTORTEC/distortos,CezaryGapinski/distortos,CezaryGapinski/distortos,DISTORTEC/distortos,jasmin-j/distortos,CezaryGapinski/distortos,jasmin-j/distortos,jasmin-j/distortos,DISTORTEC/distortos,jasmin-j/distortos