net: ipv6: Add callbacks declarations to get access to IPv6 variables

Set up callbacks for main IPv6 variables ip6add, serverip6, gatewayip6
and set options to them in flag file. These variables are often set up by
users.

Signed-off-by: Viacheslav Mitrofanov <v.v.mitrofanov@yadro.com>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>