net: ipv6: Add ip6addr, gatewayip6, serverip6 variables callbacks

Implement actions on ip6addr, gatewayip6, serverip6 varaibles.
on_ip6addr - convert IPv6 string addr to struct ip6_addr
on_gatewayip6 - convert IPv6 string addr to struct ip6_addr
on_serverip6 - convert IPv6 string addr to struct ip6_addr

Series-changes: 3
- Removed memory allocation
- Substituted -1 for error code

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>