Allow relative paths for callback include files in config.yaml

[BoergeSt]

Using absolute paths for callback includes is too rigid and forces a structure similar to the gateways by automation-One.
A simple fix would be, to allow callback files to lie relative to the given config.yaml.

There is one catch: The code has to follow symlinks to their destination, since the recommended usage is to let a symlink at a static position point towards the actual config.yaml file, and the callback needs to therefore also be relative to this location.