build: Make create_build_dir PHONY and add order-only deps

Mark create_build_dir as PHONY so directory creation is never skipped.
Also add html_data.h as order-only prerequisite to the .c pattern rule,
fixing another the race where httpd/httpd.c and httpd/page_impl.c are
compiled before the generated header exists.

Makefile

@@ -110,10 +110,10 @@ distclean:
 	-rm -f html_data.c html_data.h $(VERSION_HEADER)
 	-rm -rf $(BUILDDIR)
 
-$(BUILDDIR)/%.rel: %.c
+$(BUILDDIR)/%.rel: %.c | create_build_dir html_data.h
 	$(CC) -MMD $(CC_FLAGS) -o $@ -c $<
 
-$(BUILDDIR)/%.rel: %.asm
+$(BUILDDIR)/%.rel: %.asm | create_build_dir
 	${ASM} ${AFLAGS} -o $@ $<
 #	mv -f $(addprefix $(basename $^), .lst .rel .sym) .
 
@@ -132,7 +132,7 @@ $(BUILDDIR)/rtlplayground-$(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)
+.PHONY: clean all $(SUBDIRS) $(VERSION_HEADER) create_build_dir
 
 .PHONY:
 machine_check: