epoxy_client should construct URL for epoxy.mrom

[stephen-soltesz]

Today, updaterom.sh performs the logic of parsing /proc/cmdline, finding the epoxy.mrom prefix URL, discovering the device model, and building a full URL to download a new mrom image before burning it.

The URL discovery and construction logic should be executed by epoxy_client. And epoxy_client action should be able to specify a URL variable as:

{{kargs `epoxy.mrom_url`}}/{{.vars.device}}/{{.vars.hostname}}.mrom

Then download that image, and pass the local file to a modified updaterom.sh script that accepts a rom file as parameter.

With this support updaterom.sh can be replaced with flashrom.sh.