build: store sha256_unsigned in JSON

Introduce `sha256_unsigned` which is a checksum of the image _before_ a
signature is attached. This is helpful to compare image reproducibility.

Since the `.sha256sum` file is located in the $(KDIR) folder, switch
$(BIN_DIR) with $(KDIR) to simplify the code. The value of $(BIN_DIR)
itself is not stored inside the resulting JSON file, so it can be
replaced.

Signed-off-by: Paul Spooren <mail@aparcar.org>

scripts/json_add_image_info.py

@@ -11,8 +11,8 @@ if len(argv) != 2:
     exit(1)
 
 json_path = Path(argv[1])
-bin_dir = Path(getenv("BIN_DIR"))
-file_path = bin_dir / getenv("FILE_NAME")
+file_path = Path(getenv("FILE_DIR")) / getenv("FILE_NAME")
+
 
 if not file_path.is_file():
     print("Skip JSON creation for non existing file", file_path)
@@ -37,7 +37,14 @@ def get_titles():
 
 
 device_id = getenv("DEVICE_ID")
-file_hash = hashlib.sha256(file_path.read_bytes()).hexdigest()
+hash_file = hashlib.sha256(file_path.read_bytes()).hexdigest()
+
+if file_path.with_suffix(file_path.suffix + ".sha256sum").exists():
+    hash_unsigned = (
+        file_path.with_suffix(file_path.suffix + ".sha256sum").read_text().strip()
+    )
+else:
+    hash_unsigned = hash_file
 
 file_info = {
     "metadata_version": 1,
@@ -52,7 +59,8 @@ file_info = {
                 {
                     "type": getenv("FILE_TYPE"),
                     "name": getenv("FILE_NAME"),
-                    "sha256": file_hash,
+                    "sha256": hash_file,
+                    "sha256_unsigned": hash_unsigned,
                 }
             ],
             "device_packages": getenv("DEVICE_PACKAGES").split(),