Refactor prep.sh to allow independent OMERO and Bio-Formats releases  (rebased onto develop)



This is the same as gh-21 but rebased onto develop.

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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.