Support DHCPV6_BOOTFILE_PARAM

[bluecmd]

We should read the configuration file from a DHCPv4 option like we do today, but also a DHCPv6 option like DHCPV6_BOOTFILE_PARAM.

DHCPV6_BOOTFILE_PARAM seems perfect for us from RFC 5970:

3.2. Boot File Parameters Option

This option is sent by the server to the client. It consists of
multiple UTF-8 ([RFC3629]) strings. They are used to specify
parameters for the boot file (similar to the command line arguments
in most modern operating systems). For example, these parameters
could be used to specify the root file system of the OS kernel, or
the location from which a second-stage boot-loader program can
download its configuration file.

[bluecmd]

For v4 this might be an option to support? https://tools.ietf.org/html/rfc5071#section-4
EDIT: it is reserved for pxelinux, so probably hard to make the ipxe folks think its a good idea to piggy-back on it. Let's just keep it at 129.