also store metadata alongside content

[cgwalters]

Today we have a singular /boot/bootupd-state.json. It seems better to at least also store state alongside its content, so we'd have e.g. /boot/efi/EFI/bootupd.json as well as serializing a bit of our metadata into the BIOS partition, etc.

This would allow us to better detect situations where e.g. we happen to find an ESP from a different drive than the one holding /boot. The additional storage cost here would be minimal.