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.