netboot=xnba is deprecated, but profile-based node definitions, the x86
node templates of mkdef --template, the e1350 noderes table and the
SoftLayer importer still gave new x86 nodes xnba. They now give
netboot=ipxe, as node discovery does. Existing nodes keep their method,
and a profile change keeps netboot=xnba on a node that has it: moving a
node to the upstream loader is the choice of the site.