net: ipv6: Add IPv6 basic primitives

This patch is a collection of basic primitives that are prerequisite for
further IPv6 implementation.

There are structures definition such as IPv6 header, UDP header
(for TFTP), ICMPv6 header. There are auxiliary defines such as protocol
codes, padding, struct size and etc. Also here are functions prototypes
and its empty implementation that will be used as API for further patches.
Here are variables declaration such as IPv6 address of our host,
gateway, ipv6 server.

Series-changes: 3
- Added functions and structures descriptions
- Removed enums ND_OPT_*. It will be moved into further patches
- Substituted -1 for error codes

Series-changes: 4
- Changed functions and structures description style

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>