Allow creating persistent volume onto a separate device

For added plausible deniability I suggest adding the option of being able to install the persistent volume on a separate device other than the one running the OS.

This way Tails OS mass storage device 1 is physically different from Tails User Data mass storage 2. User data may be stored on easily concealable/disposable/hidable MicroSD card ; if properly concealed an oppponent cannot prove nor disprove whether Tails was used with or without persistent user data.

Boot sequence could look for external /home directory then revert to internal temporary /home if none are found.

Related issues

• Related to #5929
• Related to #6853 (closed)
• Related to #15662 (closed)
• Has duplicate #5561 (closed)
• Blocked by #8935

Original created by @porcino999 on 6621 (Redmine)