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It worked. It took some trial and error because I was missing something btrfs related, but here is a cleaned up version of my steps:


# If the kernel has btrfs as external module (turns out I did not need this step):
  #Insert a line to /etc/initramfs-tools/modules containing:  "btrfs"

  #make new uInitrd with btrfs module:
  mkinitramfs -c gzip -o ./initrd `uname -r`
  mkimage -C none -A arm -O linux -T ramdisk  -a 0 -e 0 -n initramfs -d ./initrd uInitrd
  rm -f ./initrd

# now insert SD into another device like a PC

# create a new btrfs partition for root

#create subvolume for root
  btrfs sub create @

#move everything in "/"  to @ except for /boot which remains ext4

# mkdir /boot on the NEW btrfs  root filesystem

# move everything from originalext4fs/boot on the OLD ext4 filesystem up one level to originalext4fs/

# from the root of the OLD ext4 ( originalext4fs/ )
  ln -s . boot

# use "btrfs filesystem show" or whatever to find the new uuid

# edit/update /boot/boot.cmd
  #change root=??? to root=UUID=YourNewBtrfsUUID
  #change to rootfstype=btrfs
  #add rootflags=subvol=@

# generate new boot.scr:
  apt-get install u-boot-tools  # if not installed already
  mkimage -C none -A arm -T script -d ./boot.cmd ./boot.scr

# edit fstab to look something like this:

  #old ext4 now /boot:
  /dev/mmcblk0p1 /boot ext4 defaults,noatime,nodiratime,commit=600,errors=remount-ro 0 1
  #new btrfs root :
  UUID=newuuid-for-btrfs / btrfs subvol=@,defaults,noatime,nodiratime 0 1
  # no change:
  tmpfs /tmp tmpfs defaults,nosuid 0 0
  /var/swap none swap sw 0 0


#reboot and see if you can do the happy dance like me!
 

It worked. It took some trial and error because I was missing something btrfs related, but here is a cleaned up version of my steps:


# If the kernel has btrfs as external module (turns out I did not need this step):
  #Insert a line to /etc/initramfs-tools/modules containing:  "btrfs"

  #make new uInitrd with btrfs module:
  mkinitramfs -c gzip -o ./initrd `uname -r`
  mkimage -C none -A arm -O linux -T ramdisk  -a 0 -e 0 -n initramfs -d ./initrd uInitrd
  rm -f ./initrd

# now insert SD into another device like a PC

# create a new btrfs partition for root

#create subvolume for root
  btrfs sub create @

#move everything in "/"  to @ except for /boot which remains ext4

# mkdir /boot on the NEW btrfs  root filesystem

# move everything from originalext4fs/boot on the OLD ext4 filesystem up one level to originalext4fs/

# from the root of the OLD ext4 ( originalext4fs/ )
  ln -s . boot

# use "btrfs filesystem show" or whatever to find the new uuid

# edit/update /boot/boot.cmd
  #change root=??? to root=UUID=YourNewBtrfsUUID
  #change to rootfstype=btrfs
  #add rootflags=subvol=@

# generate new boot.scr:
  apt-get install u-boot-tools  # if not installed already
  mkimage -C none -A arm -T script -d ./boot.cmd ./boot.scr

# edit fstab to look something like this:

  #old ext4 now /boot:
  /dev/mmcblk0p1 /boot ext4 defaults,noatime,nodiratime,commit=600,errors=remount-ro 0 1
  #new btrfs root :
  UUID=newuuid-for-btrfs / btrfs subvol=@,defaults,noatime,nodiratime 0 1
  # no change:
  tmpfs /tmp tmpfs defaults,nosuid 0 0
  /var/swap none swap sw 0 0


#reboot and see if you can do the happy dance like me!