* Occasionally, it is necessary to symbolically link the pi-gen/work
directory to a different place on the filesystem. This is true if
the current partition is low on space, or when building within
vagrant virtual machine shared mounts which do not support all
regular fs operations.
* Without this change, the pi-gen build will fail to unmount because
the OS 'mount' command returns canonical paths which fails to match
the symbolically linked path to 'work' dir.
David C Wang