We don't need two (and half) templating systems anymore, yay!
I'm keeping the changes minimal, to make the diff manageable. Some enhancements
due to a better templating system might be possible in the future.
For handling of '## ' — see the next commit.
This is the counterpiece to the boot counting implemented in
systemd-boot: if a boot is detected as successful we mark drop the
counter again from the booted snippet or kernel image.