net: netconsole: move netconsole_target config_item to config_group

In order to support a nested userdata config_group in later patches,
use a config_group for netconsole_target instead of a
config_item. It's a no-op functionality-wise, since
config_group maintains all features of a config_item via the cg_item
member.

Signed-off-by: Matthew Wood <thepacketgeek@gmail.com>
Reviewed-by: Breno Leitao <leitao@debian.org>
Signed-off-by: David S. Miller <davem@davemloft.net>