[Makefile]: add objects to clean target

Some demos will use object files from a different directory and
those files would not be cleaned by the simple CLEAN rule. This
small change insures that those objects and their dependency files
are also cleaned.

examples/rules.mk

@@ -201,7 +201,7 @@ endif
 
 clean:
 	@#printf "  CLEAN\n"
-	$(Q)$(RM) *.o *.d *.elf *.bin *.hex *.srec *.list *.map generated.*
+	$(Q)$(RM) *.o *.d *.elf *.bin *.hex *.srec *.list *.map generated.* ${OBJS} ${OBJS:%.o:%.d}
 
 stylecheck: $(STYLECHECKFILES:=.stylecheck)
 styleclean: $(STYLECHECKFILES:=.styleclean)