minor: allow passing custom assets to start SB protected CM4

[aethernet]

This PR adds the ability pass a folder with second stage boot assets as parameter when initiating usbbootscanner.

This is to allow end-user to provide config.txt and signed boot.img required to get a secureboot-enabled CM4 to attach as a mass-storage device and be reflashed.

This mostly only expose the changes made in balena-io-modules/node-raspberrypi-usbboot#53 in examples.

[alexgg]

lgtm with a small nitpick - however let's wait for Akis to review.

[dfunckt]

Is this example file meant to used directly?

[aethernet]

For demo purposes mostly
I do use it (also multi-destination) as a poor man CLI

[dfunckt]

Yeah, that's the reason I'm asking -- if it's meant to be relied upon, then it should be a proper script shipped with the package and not merely an example (which is not really maintained or tested that it works).

[aethernet]

If your concern is that would become de-facto production, it shouldn't. We still need a proper place to hook this up (the CLI, a new etcherCLI, a proper script in this repo or somewhere else)

[dfunckt]

Looks good to me, once Alex's comments are addressed feel free to merge

[alexgg]

Just a bunch of nitpicks so I would ask you to self approve if it was possible - let me know if they are not worth addressing and I will approve.