Extend the behavior of LCG_INSTALL_PREFIX as a PATH

The parameter LCG_INSTALL_PREFIX can be now interpreted as a list of prefixes of 'LCG' package installations areas. They should be ';' separated to be interpreted as a CMake list directly. For example:

-DLCG_INSTALL_PREFIX=/cvmfs/sft.cern.ch/lcg/releases\;/cvmfs/atlas.cern.ch/releases

Also added a new parameter LCG_ADDITIONAL_REPOS to add additional web repositories for binary tarfiles.