In case a user or old software created the first_boot flag file under
/boot/coreos instead of /boot/flatcar, the update breaks because /boot/coreos
exists and is prefered ofer /boot/flatcar when the kernel is written.
Make updating more robust by checking if the kernel binaries exist which as least
allows the user to create /boot/coreos/first_boot without breaking the system.

How to use

See flatcar/scripts#68

Testing done

Download the qemu image for arm64 from the linked PR above (because the amd64 did reuse the binary package):

$ wget https://storage.googleapis.com/flatcar-jenkins/developer/boards/arm64-usr/2345.3.1+dev-flatcar-master-449/flatcar_production_qemu_uefi_image.img.bz2
$ lbunzip2 flatcar_production_qemu_uefi_image.img.bz2
$ wget https://storage.googleapis.com/flatcar-jenkins/developer/boards/arm64-usr/2345.3.1+dev-flatcar-master-449/flatcar_production_qemu_uefi_efi_code.fd
$ wget https://storage.googleapis.com/flatcar-jenkins/developer/boards/arm64-usr/2345.3.1+dev-flatcar-master-449/flatcar_production_qemu_uefi_efi_vars.fd
$ wget https://storage.googleapis.com/flatcar-jenkins/developer/boards/arm64-usr/2345.3.1+dev-flatcar-master-449/flatcar_production_qemu_uefi.sh
$ chmod +x flatcar_production_qemu_uefi.sh

1. Boot up the VM and enter via SSH
2. sudo sh -c 'echo GROUP=alpha > /etc/flatcar/update.conf'
3. mkdir /boot/coreos; touch /boot/coreos/first_boot
4. cp /usr/share/flatcar/release /tmp; sudo mount --bind /tmp/release /usr/share/flatcar/release
5. set FLATCAR_RELEASE_VERSION=0.0.0 in /usr/share/flatcar/release
6. sudo systemctl restart update-engine
7. update_engine_client -update
8. ls /boot/coreos should not have a kernel