Fix rauc hook to restore machine-id in new grub environment

[claplace]

Without a machine-id, the next boot will be considered as a first boot, and any external data disk will be disabled.

Fixes #3247.

After installing my raucb update bundle and a reboot:

Jun 29 16:34:38 homeassistant kernel: Kernel command line: BOOT_IMAGE=(hd0,gpt4)/Image root=PARTUUID=a3ec664e-32ce-4665-95ea-7ae90ce9aa20 rootwait zram.enabled=1 zram.num_devices=3 systemd.machine_id=6c631b4986cb426f9b7860df4ce10629 fsck.repair=yes console=ttyS0 console=tty0 rauc.slot=B

The machine-id has been conserved 👍

[home-assistant]

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[home-assistant]

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Stale

[claplace]

I'd like to add the rel-x label, but I'm not sure I can do that myself.

[sairon]

Thanks for looking into this, you got it right. In fact, the same issue started to manifest in latest dev builds for OVA/x86 as a result of #3435. However, I'll prefer to do it differently - instead of amending the grubenv distributed in the update bundle, preserve it as whole - in the same way as we do with the *.txt files (which we use for persisting machine-id on non-grub platforms).

[claplace]

Hi @sairon, that works even better, thank you!

[sairon]

Looking good now, thank you!

[sairon]

Tagging with generic-aarch64 only, since x86/OVA targets were not affected until the patch that landed just recently only in dev.