improve variant selection

since eagle 7.x the variant handling is changed and it is not possible to select the variant with eagle_bom.py

Atm I am able to select different variants with eagle_bom.py but the output is always the same. I am also able to put non existing variant names in the variant parameter. If it is not possible to update eagle_bom.py to work with the variants at least an error would be nice.
