package/leafnode2: fix news user check

--enable-runas-user use 'news' as default but the configure stop
if news doesn't exist on the build host.
Use 'root' while cross-compiling

Signed-off-by: Romain Naour <romain.naour@openwide.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>