Makefile: have clean and distclean recurse into the tool directory

make clean left tools/output in place, so the host tools were never
rebuilt after a clean. SUBDIRSCLEAN, which builds the per-directory
clean target names for exactly that purpose, was defined but nothing
consumed it. Give clean and distclean those targets as prerequisites
and add the rule that runs the subdirectory's own clean, which
tools/Makefile already provides.

Makefile

@@ -103,14 +103,17 @@ httpd: html_data.h
 $(SUBDIRS):
 	$(MAKE) -C $@
 
-clean:
+clean: $(SUBDIRSCLEAN)
 	-rm -f html_data.c html_data.h $(VERSION_HEADER)
 	-if [ -d $(BUILDDIR) ]; then find $(BUILDDIR) -type f ! -name "*.bin" -delete; fi
 
-distclean:
+distclean: $(SUBDIRSCLEAN)
 	-rm -f html_data.c html_data.h $(VERSION_HEADER)
 	-rm -rf $(BUILDDIR)
 
+$(SUBDIRSCLEAN):
+	$(MAKE) -C $(@:clean=) clean
+
 $(BUILDDIR)/%.rel: %.c | create_build_dir html_data.h
 	$(CC) -MMD $(CC_FLAGS) -o $@ -c $<
 
@@ -133,7 +136,7 @@ $(BUILDDIR)/rtlplayground-$(FILENAME_EXTENSION).bin: $(BUILDDIR)/rtlplayground.i
 	tools/output/crc_calculator -u $@
 	ln -sf $(MACHINE)/rtlplayground-$(FILENAME_EXTENSION).bin output/rtlplayground.bin
 
-.PHONY: clean all $(SUBDIRS) $(VERSION_HEADER) create_build_dir
+.PHONY: clean distclean all $(SUBDIRS) $(SUBDIRSCLEAN) $(VERSION_HEADER) create_build_dir
 
 .PHONY:
 machine_check: