When booting from DVD, if USB stick plugged, Tails ends up running from USB stick

I have known about this issue since a while from frontdesk. I just realised there is no ticket about it, or maybe we always thought of it as an undocumented feature.

Steps to reproduce:

- Boot Tails from DVD (lets say Tails 1.6)

- Once booting started, plug USB stick with another Tails version (lets say Tails 1.7)
- Check Applications/Tails/About Tails

The Tails version is the USB stick one.

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One user that cannot boot from USB stick reported using this as a workaround to boot Tails, and use Persistence and automatic upgrades from the USB stick._

Related issues

Is duplicate of #7475

Original created by @emmapeel on 10944 (Redmine)

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