[PFKEYV2]: CONFIG_NET_KEY_MIGRATE option

Add CONFIG_NET_KEY_MIGRATE option which makes it possible for user
application to send or receive MIGRATE message to/from PF_KEY socket.

Signed-off-by: Shinta Sugimoto <shinta.sugimoto@ericsson.com>
Signed-off-by: Masahide NAKAMURA <nakam@linux-ipv6.org>
Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: David S. Miller <davem@davemloft.net>