Remove the hard-coded version number from the PDF documentation link

Use the /latest/*manual-pdf link for the OMERO and Bio-Formats instead. This
allows a patch release such as 4.4.8.1 to keep pointing at the 4.4.8 PDF
documentation (assuming the redirected links are not updated).
Also remove the PDF_URL replacement key in gen.py and bfgen.py
Refactor prep.sh to allow independent OMERO and Bio-Formats releases

Test the emptiness of the OMERO_BUILD and BIOFORMATS_BUILD variables to
determine the components to be prepared for release.
For each components, the artifacts checking, the release directory creation
and the artifacts symlinking are conditioned by the existence of the
corresponding build variable.
Add missing echo statements
Merge pull request #21 from sbesson/omero_bioformats_split

Refactor prep.sh to allow independent OMERO and Bio-Formats releases
Merge pull request #20 from sbesson/pdf_manual_link

Remove the hard-coded version number from the PDF documentation link