[IPV6]: Add Kconfig to enable Mobile IPv6.

Add Kconfig to enable Mobile IPv6.
Based on MIPL2 kernel patch.

Signed-off-by: Noriaki TAKAMIYA <takamiya@po.ntts.co.jp>
Signed-off-by: Masahide NAKAMURA <nakam@linux-ipv6.org>
Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>