net: dhcp6: Add DHCPv6 (DHCP for IPv6)

net: dhcp6: Add DHCPv6 (DHCP for IPv6)

Adds DHCPv6 protocol to u-boot.

Allows for address assignement with DHCPv6 4-message exchange
(SOLICIT->ADVERTISE->REQUEST->REPLY).  Includes DHCPv6 options
required by RFC 8415.  Also adds DHCPv6 options required
for PXE boot.

Possible enhancements:
- Duplicate address detection on DHCPv6 assigned address
- IPv6 address assignement through SLAAC
- Sending/parsing other DHCPv6 options (NTP, DNS, etc...)

Signed-off-by: Sean Edmond <seanedmond@microsoft.com>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>