Implement flash backup/restore commands

[doegox]

Implement commands to ease a complete flash backup & restore.
It would allow to preserve and reinstall config and data easily after some experiments or broken upgrade/downgrade.
Backup should associate fw version info so a fw should refuse a backup made from a later version. (earlier version should be ok and migrated by the fw if needed)
Then up to the client to store the retrieved backup in a file, but all clients should adopt the same format.
E.g. zip file with tree based on FDS: FILE_ID => directory name, RECORD_KEY => filename. And raw data in file.