[do_build.sh] Remove optional steps...

... to minimize build breakage.
- nilfvm does nothing.
- copy requires extra configuration lines that are not yet documented, or it tries to copy stuff to /master, which breaks the build for most users.
- extra_pkgs can be replaced by just manually building the extra packages needed, which widely vary between users anyway.
- packages_tree also requires extra configuration. That step is probably not needed for most users either.