Ticket #147 (new defect)

Description

All build-steps allow a 'workdir' to be explicitely specified, overriding the default value from the builder. However, the logic that decides whether to use the default or not simply checks "if workdir:...". This means that a value of won't work; the user needs to explicitely specify './' to get that effect. The obvious fix is to replace the logic "if workdir" by "if workdir is None".