net: ipv6: Enable IPv6 typeconversion specifier

Add the possibility to recognize IPv6 address in print function.
To output IPv6 address use %pI6 specifier.

Series-changes: 3
- Substituted #if (...) for if (...) to get better readability

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>