Optimize SquashFS for currently supported use cases

We still optimize (sort) the SquashFS for bare metal DVD, while nowadays:

ISO images are supported for VMs only.
Everyone else should use the USB image.

So let’s instead optimize for bare metal USB sticks, i.e. the most common real-world use case. I doubt it’ll harm performance for VMs (an internal hard drive is very different from a bare metal DVD drive anyway).

Related issues

Blocks #16209

Original created by @intrigeri on 16946 (Redmine)

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