install to-filesystem: support opt-out for the `root` and `boot` mountspec kernel arguments

[jbtrystram]

I am working on using bootc install to-filesystem to create disk images for Fedora CoreOS.

Currently this works with theses osbuild changes, but the resulting disk image have root=UUID... and boot=UUID... kernel arguments baked in.

This causes issues because ignition generates new uniques UUIDs for those filesystems during the first boot, and if the root argument is already present, some of that logic get short-circuited:

https://github.com/coreos/fedora-coreos-config/blob/bf40cb9f3eef371434b2f6e625250185af7bbc3d/overlay.d/05core/usr/lib/dracut/modules.d/35coreos-ignition/coreos-diskful-generator#L77-L79

So it would be good to be able to not inject those kargs and have our intramfs bit do it after the UUID are regenerated.

More discussion in coreos/fedora-coreos-tracker#1827

[cgwalters]

Did you see this coreos/fedora-coreos-tracker#1827 (comment) ?

[jbtrystram]

Did you see this coreos/fedora-coreos-tracker#1827 (comment) ?

Yes, I tried playing with it but the only way to get the image to boot is to remove the karg entirely.
I guess we could have a dumb-firstboot placeholder value and look for it, but I suppose bootc won't be happy if the mountspec is invalid?

[cgwalters]

Oh right yes, we should support omitting the kargs entirely. We'll want that for DPS based too. Hmm, maybe we just make --root-mount-spec= i.e. the empty string mean to omit it.

[dustymabe]

Oh right yes, we should support omitting the kargs entirely. We'll want that for DPS based too. Hmm, maybe we just make --root-mount-spec= i.e. the empty string mean to omit it.

This SGTM!

[cgwalters]

Fixed by 560711d