Pass a "FREEBSD" user-class in PXE dhcp request

[bapt]

Summary

rfc3004 allows to pass multiple user classes on dhcp requests
this is used by dhcp servers to differentiate the caller if needed.

Test Plan

Usage example with isc-dhcp

if exists user-class and option user-class = "FREEBSD" {
   option root-path "tftp://192.168.42.1/FreeBSD;
}

[bapt]

Use the defined value

[tsoome]

LGTM.