ZTP: IPv6

This feature introduces ZTP-over-IPv6 support with DHCPv6 clients as part of the default configuration on the device. During the bootstrap process, the device first uses the DHCPv4 client to request for information regarding image and configuration file from the DHCP server. The device checks the DHCPv4 bindings sequentially. If there is a failure with one of the DHCPv4 bindings, the device continues to check for bindings until provisioning is successful. If there are no DHCPv4 bindings, however, the device checks for DHCPv6 bindings and follows the same process as for DHCPv4 until the device can be provisioned successfully. The DHCP server uses DHCPv6 options 59 and 17 and applicable sub-options to exchange ZTP-related information between itself and the DHCP client.