Draft: rescue-mode: Try to mount btrfs root subvolume for Debian and other distributions

Minimal effort to fix #1102604, see discussion in https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1102604

Marked draft because I am not sure if d-i should support other distribution subvolume schemes. If yes, any other that should be added ?

If it is agreed to support them all, I may use a for loop instead of duplicating the mount command.