net-snmp: enable ipv6 support

Add packageconfig for ipv6 and enable it when it's defined in distro_features.

Signed-off-by: Zhu Yanjun <yanjun.zhu@windriver.com>
Signed-off-by: Yue Tao <Yue.Tao@windriver.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
Signed-off-by: Armin Kuster <akuster808@gmail.com>