Log Message:
pkgtools/createbuildlink: use SUBST instead of ad-hoc sed

This allows to show the actual changes by setting SUBST_SHOW_DIFF=yes,
and with SUBST_NOOP_OK=no it also demonstrates that no substitutions are
needed for substplistbasedirs.

Another thing this demonstrates is this wrong patch:

-echo >>$sedrules "s|@@PKGVERSION@@|@PKGVERSION@|g"
+echo >>$sedrules "s|@3.17@|3.17|g"