Add device & model names to kargs for epoxy

[stephen-soltesz]

The stage1-template.ipxe script and however we generate stage1 images for legacy hardware, we should encode some known hardware information into the kargs so that they are available to epoxy during boot.

In particular, this could be helpful for updating the mlx rom. For example, if the kargs include:

epoxy.model=Mellanox epoxy.device=ConnectX3

Then the process that downloads and flashes the rom could construct a URL to locate the latest image, e.g.

https://storage.googleapis.com/epoxy-mlab-staging/roms/{{.kargs.epoxy.model}}/{{.kargs.epoxy.device}}/{{.kargs.epoxy.hostname}}.mrom