Fix setup problems due to using old debian

by getting the files and keys from the archive etc.

Substitute the use of cd in create_image.sh which caused a bug that
made the generated image not always be placed in the correct dir
depending on if the debootstrap-dir has already been created or not.

Create setup.sh so that it can be called directly after creating
the new image without having to copy/paste from the README.

Also add a troubleshooting section to README concerning some of the
problems above and more.