Build AMD64/ARM64 netboot images for ChromeOS devices

This MR adds netboot images that can be used on ChromeOS devices via their non-standard boot flow. They will fail to boot on older devices (pre-2019) due to hard-coded size limits in the firmware, but should be fine for more recent boards (which may need hardware enablement work like ordinary computers).

Unfortunately my arm64 chromebook is old enough to not boot this. My x86 chromebook's display doesn't work with this, but I can switch to VT2 and run poweroff to power it off, so I can say that it boots.

I'm not sure how best to handle the low size limit on the older boards, need to think and experiment more.

changed the description

Hi, I suppose it wouldn't make sense to try and have a common place for depthcharge support, as both platforms are going to need specific handling (esp. given your latest report about max sizes and so on), so it makes sense to have similar but different snippets in various arch files?

I could put the commands in something like util/gen-depthcharge-disk and call that instead, but I don't see how I can make things more general than that. I still have trouble understanding this repo, though.

I've tried to encapsulate all board-specific things into depthchargectl anyway, so the differences are mostly going to be the board argument, input files (kernel, initrd, and dtbs for arm), disk/partition size. At worst, maybe a second partition like in concatenable netboot images.

Oh that was just out of curiosity, food for thoughts, etc. If you're happy with the current state of affairs, that can be merged!

merged