packaging: configuration depending variables

- removed unused INTERFACE_VERSION from config.py
- added SYSCONFDIR and DATADIR to config.py
- config.py is created at make execution instead of at
  configure execution. It is possible to change prefix
  passing it to make.
- removed unused LOCALSTATEDIR from FileLocations
- in FileLocations build paths using config.py variables
  for paths depending by configuration

Change-Id: I4a68802381480bf8de7547c2e66ccb817a35b857
Signed-off-by: Sandro Bonazzola <sbonazzo@redhat.com>