CI: Update test/nokia_rx51_test.sh to use prebuilt images

Now that the Dockerfile creates images which have the binaries we
require included, have CI make symlinks for them and update the existing
script to support this.

Signed-off-by: Tom Rini <trini@konsulko.com>
Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Simon Glass <sjg@chromium.org>