Integrate arm64 kernels like the armhf ones
On armhf/imx6/nitrogen kernel updates happen automatically via
- https://source.puri.sm/Librem5/image-builder/blob/master/data/update-zImage-link
- https://source.puri.sm/Librem5/image-builder/blob/master/root.sh#L43
This was done differently for arm64/devkit:
The /etc/kernel/postinst.d/
logic should be adjusted to handle arm64/devkit as well.