net: ipv6: Add IPv6 build options

Add options to Makefile and Kconfig file to build IPv6

Series-changes: 3
- Added help for IPv6 support

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>