7681 uts: nfs_dlboot fall back to use rootpath from dhcp
The traditional solaris behavior is to fetch root path, server name etc from DHCP vendor extensions. However, it is not always easy or even possible to set vendor extensions, so we can also have fall back to use standard dhcp root path option (17). Sure, we still also do have an alternative to pass the data to kernel by properties (server-ip, server-name, server-path), but using properties would require a) knowing about them, b) additional effort.
However, there is an little catch, the option 17 does not describe what exactly is root path, so we do check serverip:/path and set both server_ip and path if needed.