package/tar: add support for acl and xattr

If acl or attr is going to be installed, add support for them to tar.

[Thomas: explicitly disable acl and attr support when the
corresponding packages are not available.]

Signed-off-by: Maxim Mikityanskiy <maxtram95@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>