Run eos-firstboot a second time on existing installs

[wjt]

An alternative approach to #408.

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In new dual-boot installs of 6.0.0, eos-firstboot did not correctly grow the filesystem due to the bug fixed in commit
56aea35.

We now want to run it a second time. Ideally we would only trigger it on affected dual-boot systems; but it is harder to detect that case than to run eos-firstboot again and have resize2fs gracefully do nothing.

While we could easily run a unit only on dual-boot systems with:

ConditionKernelCommandLine=endless.image.device
ConditionKernelCommandLine=!endless.live_boot

to do this only once requires a second unit with its own new flag file. So rather than introducing a second flag file, let's harmlessly re-run this unit once on all systems, and take the opportunity to move the flag file out of the top-level of /var to /var/lib/eos-firstboot.

https://phabricator.endlessm.com/T35432

[wjt]

Draft because untested.

I also have a mild concern that there may exist other devices out there where the filesystem and partition don't match, and so unexpectedly running resize2fs may not in fact be harmful. But I'm struggling to think of a non-contrived situation where this would happen & be a problem.

[wjt]

I tested this on a dual-boot system in the state where the partition had been resized but not the filesystem; as expected, the job re-ran once and grew the filesystem.

[dbnicholson]

I also have a mild concern that there may exist other devices out there where the filesystem and partition don't match, and so unexpectedly running resize2fs may not in fact be harmful. But I'm struggling to think of a non-contrived situation where this would happen & be a problem.

I paused a bit on this. If we do run into a situation where old systems can't run the resize2fs sequence again, we can change the script like:

need_resize=false
[ -f /var/lib/eos-firstboot ] || need_resize=true
[ -f /var/eos-booted ]] && relevant_dual_boot_setup && need_resize=true
if "$need_resize"; then
    resize2fs "${root_part}"
fi

> /var/lib/eos-firstboot