YBD gets confused if not run from definitions base directory

Created by: leeming

Target seems to be calculated with the following target = os.path.join(config['defdir'], config['target']) However, if 'defdir' is not set in a config file, CWD is used instead.

This causes issues if trying to run ybd like the following python ybd.py ../../baserock/definitions/systems/minimal-system-x86_64-generic.morph x86_64