Woof code changes

Most of these changes only apply to SD image builds, except 'Add a ../woof-output directory'.

I do my builds in a 4GB loopback mounted ext2 image file on a FAT partition, and it is impossible to expand a 4GB SD card image inside of a 4GB ext2 image file.  By unpacking  the SD image into ../woof-output, and making woof-output a symlink to a directory on the FAT partition I can build inside the ext2 image file without hard-coding the path to the SD image file into 3builddistro.
I haven't tested the Raspbian build without these changes, but I kept them separate because I don't think they are necessary if building on a linux partition with plenty of room.