Ensure enough entropy is available when setting up persistence

Copying from the description of #11897:

Given I just installed Tails on a USB stick
When I boot it for the first time
Then I want to have good entropy to create the persistent storage

This was supposed to be solved by #11897, but now that our recommended way to install Tails on a USB stick is via the USB Image instead of the Tails Installer, #11897 won’t help much to solve this.

Related issues

Related to #11897

Original created by @segfault on 16891 (Redmine)

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