Resurrect the base Ubuntu tar file.

This reverts commit 29b2a1e917baba4737f9f76fd6cc0f28bbb630fa.

As it turns out, Github digested it despite the error during commit.  As
long as it's part of the repository, it might as well be part of the
tree.
Add an image for building with rt288x toolchain.
Skeletal user image definition.

Needs content in id_dsa.pub.  Builds a user specific docker image for
rt288x cross compilation.