Add Heads certified firmware option for NovaCustom V54 & V56

[andrewdavidwong]

To be merged with: QubesOS/qubes-posts#144

CC: @wessel-novacustom

[marmarek]

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[andrewdavidwong]

@maiska: FYI, when I self-assign a PR, feel free to leave it open for me to merge. Sometimes I have to wait for something else to occur before I can merge it, and sometimes I need to merge it alongside something else (e.g., QubesOS/qubes-posts#144 in this case). No harm done in this case; just thought I'd mention it for future reference. :)

(I know I can use "draft" mode for PRs, but then some reviewers interpret that to mean that the PR isn't ready for them to review yet, so we get stuck in a loop where I'm waiting for them to review it before I unmark it as a draft, while they're waiting for me to unmark it as a draft before they review it. I realized that what I really wanted to convey was, "I will merge this PR myself after it passes review," and that's probably not what the "draft" mode is for. Rather, I realized that the "assignee" and "reviewer" properties are probably more appropriate to convey this.)

[maiska]

@andrewdavidwong noted! :)