[enh] Restore from an usb key

[zamentur]

The problem

When users need to restore their system (after an sd card failure), there is no reinstall process.

Solution

This pr change the hypercube.sh to search a "backup.tar.gz" in /root or in a usb key, if it found one, it try to restore it with "yunohost backup restore" command.

PR Status

Untested, i have no internetcube here.

Todo:

• Add a hook in doctorcube to backup and restore /etc/hosts, /etc/environment, hostnamectl, AND CHANGE DEFAULT ROOT PASSWORD

COULDN'T BE RELEASED WITHOUT a solution to replace root default password

How to test

1. Use install-sd.sh to flash the internetcube image without install.hypercube and using the -w options to replace the hypercube.sh by the one of this PR 'bash install-sd.sh -w THIS/PATH/TO/hypercube.sh'
2. Put your backup tar.gz on an usb stick
3. Call it backup.tar.gz (it's an hardcoded name)
4. Plug your usb stick
5. Power on your internet cube
6. wait and try to reach the debug page (as for install)

Validation

• Principle agreement 0/2 :
• Quick review 0/1 :
• Simple test 0/1 :
• Deep review 0/1 :

[alexAubin]

Bump @zamentur ...