Depthcharge image improvements

Alper Nebi Yasakrequested to merge
alpernebbi/debian-installer:cros-better-images into master

These are some fixes and improvements for the depthcharge images that I couldn't send in time for bookworm. Most prominently, working around partman defaulting to MBR on non-UEFI x86, adding a second partition for e.g. firmware, and switching to gen-image-hd for the disk image (and adding GPT support to it in the way).

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