systemd-container: recommend tar

systemd-container: recommend tar

If systemd is configured with importd, "machinectl import-*" can be
used to add container trees under /var/lib/machines.

But "machinectl import-tar" makes use of "tar --numeric-owner" which
is not supported by the busybox binary, as shown below:

    # machinectl import-tar /tmp/container.tar.xz
    Enqueued transfer job 1. Press C-c to continue download in background.
    Importing '/tmp/container.tar.xz', saving as 'container'.
    Imported 0%.
    tar: unrecognized option '--numeric-owner'
    BusyBox v1.31.1 () multi-call binary.
    Usage: tar c|x|t [-ZzJjahmvokO] [-f TARFILE] [-C DIR] [-T FILE] [-X FILE] [FILE]...
    Failed to decode and write: Broken pipe
    Exiting.

To fix this, recommend the standard tar package.

Cc: Chen Qi <Qi.Chen@windriver.com>
Signed-off-by: Vivien Didelot <vdidelot@pbsc.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>