Provide some way to set/preserve variables directly on SD card

[bjencks]

I normally run my pis without a console attached, and only attach a serial console for debugging and setup. Unfortunately, that's the only way to modify firmware variables now, which means I have to do it every time I replace with a newer firmware. Either a tool to set the variables in the firmware image from Linux or a way to copy variables from one version to another, would let me do in place upgrades much more easily.

[garybuhrmaster]

Appears to be related to: https://github.com/pftf/RPi4/issues/6

[bjencks]

Related, yes, but not the same -- I don't need to be able to make these changes while the system is running; I do need to be able to make them directly on the files, rather than through the firmware interface.

[kpfleming]

Ditto; I'd be fine with something I can drive from Ansible to make the changes after unpacking a new firmware release.