New changes / features for composefs-backend

[Johan-Liebert1]

Finish up implementation for install-to-filesystem for composefs-backend

We were using composefs-native and composefs-backend interchangeably.
Replace all instances of composefs-native with composefs-backend

Move all composefs-backend options to a single struct so that we can
test for boolean instead of testing for Some/None for composefs-backend
options

The source imgref was being stored in the .origin file, even if target imgref was
provided. Now we store the target imgref

Start finalize-staged service on update/switch

Add composefs opts to install-to-existing-root

[Johan-Liebert1]

Currently with the option --bootloader=systemd we do everything according to the BLS spec, but the bootloader is still grub, as we don't have systemd-boot support in bootupd. Until coreos/bootupd#978 lands, I was thinking of a very crude hack of simply replacing grubx64.efi with systemd-boot.efi (renamed to grubx64.efi so shim picks it up); this is basically how I've been testing systemd-boot until now.

Does it make sense to do the above until we have systemd-boot support in bootupd?

[cgwalters]

Yes I'll try to take a look at coreos/bootupd#978 soon.

Otherwise see #806 (comment) - I think the "no bootupd = systemd-boot" path also makes sense and should be relatively straightforward to implement here.

[cgwalters]

Ah my bad, I think we have some textual (partial semantic) conflicts in #1662

[Johan-Liebert1]

That's okay. I can rebase on main afterwards