nixos/systemd-boot: use unique path for devicetree blob

Use the store directory for the devicetree package containing the
desired DTB when installing to the ESP. This allows for more than one
NixOS generation containing differing DTBs to coexist on the same ESP
(similar to how we can have multiple kernels & initrds). This change
removes the assumption that the filepath passed to `copy_from_file` is a
file that lives at the toplevel of a nix output path (which prior to the
systemd-boot DTB support was the case for the kernel and initrd
derivations).

nixos/modules/system/boot/loader/systemd-boot/systemd-boot-builder.py

@@ -33,6 +33,7 @@ CAN_TOUCH_EFI_VARIABLES = "@canTouchEfiVariables@"
 GRACEFUL = "@graceful@"
 COPY_EXTRA_FILES = "@copyExtraFiles@"
 CHECK_MOUNTPOINTS = "@checkMountpoints@"
+STORE_DIR = "@storeDir@"
 
 @dataclass
 class BootSpec:
@@ -150,12 +151,13 @@ def bootspec_from_json(bootspec_json: dict[str, Any]) -> BootSpec:
 def copy_from_file(file: str, dry_run: bool = False) -> str:
     store_file_path = os.path.realpath(file)
     suffix = os.path.basename(store_file_path)
-    store_dir = os.path.basename(os.path.dirname(store_file_path))
-    efi_file_path = f"{NIXOS_DIR}/{store_dir}-{suffix}.efi"
+    store_subdir = os.path.relpath(store_file_path, start=STORE_DIR).split(os.path.sep)[0]
+    efi_file_path = f"{NIXOS_DIR}/{suffix}.efi" if suffix == store_subdir else f"{NIXOS_DIR}/{store_subdir}-{suffix}.efi"
     if not dry_run:
         copy_if_not_exists(store_file_path, f"{BOOT_MOUNT_POINT}{efi_file_path}")
     return efi_file_path
 
+
 def write_entry(profile: str | None, generation: int, specialisation: str | None,
                 machine_id: str, bootspec: BootSpec, current: bool) -> None:
     if specialisation:

nixos/modules/system/boot/loader/systemd-boot/systemd-boot.nix

@@ -28,6 +28,8 @@ let
 
     isExecutable = true;
 
+    inherit (builtins) storeDir;
+
     inherit (pkgs) python3;
 
     systemd = config.systemd.package;