Add service to ensure systemd-boot installed

[dbnicholson]

On systemd-boot systems, the bootloader is updated with bootctl update on every boot. Unfortunately, it doesn't currently work because bootctl considers systemd-boot to be installed if /EFI/systemd is non-empty in the ESP. The image builder only populates /EFI/BOOT, so systemd-boot is not considered installed.

If bootctl does not consider systemd-boot to be installed, eos-ensure-sd-boot will check the LoaderInfo EFI variable to see if the current bootloader is systemd-boot. If so, it populates /EFI/systemd so that the next time bootctl update runs, it will update the bootloader.

https://phabricator.endlessm.com/T34703

[dbnicholson]

I tested this on both grub and systemd-boot VMs. For the grub system it correctly did nothing. For the systemd-boot VM, it correctly populated /EFI/systemd. That allowed bootctl update to run and update the EFI binaries. Unfortunately, it also tried to update /boot/loader, and that doesn't exist since we have some hacks to use a fake symlink /boot/loader.sln.

Marking this as a draft for now. One solution I can think of is to change systemd downstream so that bootctl only tries to install or update the EFI binaries and doesn't touch the rest of /boot. We've been getting along fine without any of the things it wants to put in `/boot/loader.

[dbnicholson]

I changed this to run bootctl install instead. That depends on endlessm/systemd#167 so that the installation doesn't cause potential confusion with the fake symlink hack. I tested this on a PAYG VM and it seems to work well.