support to move system to SSD/HD (btrfs replace / ? + cp -ax)

(Edit: The issue originally talked about "btrfs send".)

The trivial method to move a FreedomBox btrfs system to another storage device (like a HD or SSD) is to use "btrfs replace". And it even works while the system is running.

For system movements that involve non-btrfs filesystems (e.g. moving from an F2FS sdcard to a BTRFS HD, or from a BTRFS image to a F2FS SSD), plinth could call a script to change or boot into single-user (ro rootfs) mode and use e.g. "cp -ax" (or something better) to copy the entire root filesystem to another location, update-grub and reboot again normally.

Proof of concept commands for BTRFS are below and for F2FS at #1905 (comment 184983)

Example of a successful application by a forum member: https://discuss.freedombox.org/t/how-to-setup-raid/1401