Don't unpack extra/skeleton archives if a target is provided

Don't unpack extra/skeleton archives if a target is provided

ExtraTrees= and SkeletonTrees= are currently ambiguous when a target
location is provided. The user could either mean that the archive has
to be unpacked to the target location or the user could mean that he
wants to copy the archive to that location in the image.

We should still try to resolve the ambiguity, but for now, let's assume
that when a target is provided that the user wants to copy the archive
into the image instead of unpacking the archive to that specific location.

To make this work, we also modify the logic slightly to create only the
parent directories of the target so that cp doesn't fail because we try
to copy a file to a directory.