[IPV6]: Seperate sit driver to extra module

This patch removes the driver of the IPv6-in-IPv4 tunnel driver (sit)
from the IPv6 module. It adds an option to Kconfig which makes it
possible to compile it as a seperate module.

Signed-off-by: Joerg Roedel <joro-lkml@zlug.org>
Signed-off-by: David S. Miller <davem@davemloft.net>