classes/tar-image: allow TAR_IMAGE_DIRNAME to be empty.
classes/tar-image: allow TAR_IMAGE_DIRNAME to be empty.

If empty crate a image without the TAR_IMAGE_DIRNAME dir.

sknsean/core,hundeboll/core,sknsean/core,hundeboll/core,hundeboll/core,hundeboll/core,sknsean/core,diorcety/oe-lite-core,diorcety/oe-lite-core,sknsean/core,sknsean/core