When making release with NOPORTS, we'll checkout only the
mininal set of ports required to make the docs. However,
we also need ports/sysutils/cdrtools in order to make the
ISO images. When a platform doesn't have packages, the
release will fail in that case. Add ports/sysutils/cdrtools
to RELEASEPORTSMODULE for the DOMINIMALDOCPORTS case to
handle the NOPORTS release build.

Note that this change doesn't try to handle the NOPORTS with
NODOC case. For we have NOPORTSATALL set and it seems wrong
to check out a ports module in that case.