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.
Replace file-path prerequisite tools/output/fileadder with order-only
dependency on the tools PHONY target.
Also add tools as order-only dependency to the final .bin target which
invokes all of the tools.
Make defines defaults for commonly used variables, therefore
we need to check first how the variable was defined. If `default`
value was used, override it with our own default.
Priit Laes