Snapshot btrfs root subvolume after installation when installing on BTRFS

[serhepopovych]

It might be useful to
1) snapshot it as rootfs for containers (e.g. LXC) saving space and benefiting from CoW feature of BTRFS
2) have multiple, isolated from each other, installations with different set of packages (e.g. different desktop environments)
3) have option to recovery system by booting from "original" snapshot.

To implement this we need
1) add grub menu entry to boot from "original" snapshot for recovery purposes
2) inject finish-install.d script that executed by installer after root subvolume unmounted
to create snapshot.

[serhepopovych]

There is commit 3a86e71 ("simple-cdd-meta/debian: Disable GRUB submenu and put all entries in main") that unifies GRUB menu entry generation procedure across all Debian releases.

It is intended to help with "Snapshots ..." menu generation later when snapshots implemented.