Mirrored Boot option for systemd-boot

[Astralchroma]

For a system where redundancy is a goal, it may be preferable to have multiple EFI System Partitions across multiple drives, so that if one drive fails, there is still a usable EFI System Partition.

While this can be accomplished using mdadm using an older metadata version, it is somewhat of a hack and shouldn't be used as a go-to solution.

NixOS does provide the capacity to mirror /boot when using GRUB (see boot.loader.grub.mirroredBoots), however the option is not available for systemd-boot, which I would prefer to use.

This shouldn't be too difficult to implement given that systemd-boot's bootctl supports installing to alternate directories using the --esp-path option.

[eclairevoyant]

See #246897 for prior art
duplicate of #152155