`/boot` isn't necessarily the same as the ESP

[cole-h]

Currently, we assume that the ESP is located at /boot, even though we attempt to auto-detect its true location using the NixOS boot.loader.efi.efiSysMountPoint option. It is possible that the option is set to e.g. /boot/EFI, but our code assumes /boot is the ESP (e.g. we write to EFI/ and loader/, but as one might notice, that would create /boot/EFI/EFI in this situation).

This may require some refactoring, or it may just require renaming the --esp installer option to --boot-path (and document that it should be passed the path that has or should have EFI/ and loader/ as subdirectories).