configure: Default to automatically choose branding
configure: Default to automatically choose branding

Cockpit is meant to be the interface for the operating system it's
running on, so lets auto-select the branding by default. We can still
fall back to the "Cockpit" branding in other cases.

This lets more people get this right, rather than us having to tell
them every time.

Closes #5076
Reviewed-by: Dominik Perpeet <f7830b2723a5db4fc185058b06cfab4b89a38da9@redhat.com>

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