Use the --owner= and --group= options to make sure the entries in
the built tar file are owned by root.  Without this change, a
careless sysadmin using the tar-pkg target can easily end up
installing a kernel that is writable by the unprivileged user
account used to build the kernel.

Test that these options are understood before using them so that
non-GNU versions of tar can still be used if the operator is
appropriately cautious.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>