ReleaseData Handling Update, master branch (2019.08.02.)

While updating the analysis release Docker images I had to realise that athena.py still relies on the $CMTCONFIG environment variable to print which platform it is running on. This seemed pretty silly, so with this update I'm trying to make the handling of this metadata a bit more reasonable.

First off, the code now looks for the ReleaseData file(s) using $CMAKE_PREFIX_PATH instead of $LD_LIBRARY_PATH. The latter could lead to some weird situations in the past (a user project that does not compile any libraries could report itself as the AthenaExternals project). Since now we use CMake for all of our builds, using this variable should be safe enough.

Second, I removed the usage of the CMTCONFIG environment variable from the code. Replacing it with ${<project name>_PLATFORM} instead. So that at one point we may stop setting it all together for the offline releases.