When directives-template.xml was created in 282230882c,
this generator started picking it up. Let's filter it out properly again,
and also simply the filter while at it.
The name of the helper didn't match the name of the meson target, which was
always confusing me. With this change, we consistenly use "update" to
re-generate things which we otherwise keep in vc, and "make" for things
which are generated during each build.
