Difficulty: ★★★☆☆
In some installs /boot is an extra partition, which also is used for UEFI
- The size of this partition is too small to hold more than 1 kernel
- While Updating, everything is protected through btrfs-snapshots (but not the kernel in /boot)
Move the kernels to btrfs
- To be able to do full rollbacks including the kernels.
- without a change on any of the partitions (because of the UUID)
- To be able to boot the same way as it boots now
This all can be done in the running system, but if something goes wrong, it is handy to have a backup, and a live-USB ready 
To move the kernels into btrfs it is only necessary to change the mountpoint of the BOOT-partition, so it becomes a EFI-partition.
The following must be done as root
sudo su -
We unmount the partition
umount /boot
In /etc/fstab change the mountpoint of the vfat-partition with the editor of your choice (nano, mc, vim, …) from /boot to /boot/EFI
UUID=A2E3-0A6F /boot vfat rw,...
becomes
UUID=A2E3-0A6F /boot/EFI vfat rw,...
then reload fstab
systemctl daemon-reload
create the new mountpoint
mkdir /boot/EFI
and remount the EFI-Partition
mount /boot/EFI
Now the file grub.cfg would be in the wrong place. It has to be in /boot/grub/grub.cfg but now is in /boot/EFI/grub/grub.cgf. To make grub.cfg show up in the right place we only need a symbolic link
ln -sr /boot/EFI/grub /boot/grub
The kernels are now in /boot/EFI, but have to be in /boot to be bootable
Instead of moving, we install a new kernel. It will be installed to /boot
mhwd-kernel -i linux515
The kernel and the initramdisks have to show up in /boot. If this works, you are able to reboot.
Cleanup
Then after a reboot you can delete the kernel and initrd from /boot/EFI/
rm /boot/EFI/initram*.img
rm /boot/EFI/*.kver
rm /boot/EFI/vmlinuz-*
DONE
Do not forget to now add a second kernel 
