flatcar-update: Provide hash_sha256 attribute for OEM packages

The newer Omaha 3.1 hash_sha256 attribute is now supported by Nebraska
and should be used for OEM payloads.
Provide it in addition to the old "hash" attribute for OEM packages (the
generic payload uses the Omaha Flatcar extension with the SHA256 hash).

bin/flatcar-update

@@ -222,13 +222,17 @@ for DOWNLOAD_FILE in "$PAYLOAD" "${EXTENSIONS[@]}"; do
   SIZE=$(stat -L --printf='%s\n' "${DOWNLOAD_FILE}")
   BASEFILENAME="$(basename -- "${DOWNLOAD_FILE}" | sed 's/flatcar_test_update-//g')"
   REQUIRED="false"
+  OPTHASH256=""
   if [ "${DOWNLOAD_FILE}" = "${PAYLOAD}" ]; then
     # In case a local payload is given we have to use the correct name
     BASEFILENAME="flatcar_production_update.gz"
     REQUIRED="true"
+  else
+    HASH256=$(sha256sum -b "${DOWNLOAD_FILE}" | cut -d " " -f 1)
+    OPTHASH256="hash_sha256=\"${HASH256}\""
   fi
   tee -a /tmp/response > /dev/null <<-EOF
-	<package name="${BASEFILENAME}" hash="${HASH}" size="${SIZE}" required="${REQUIRED}"></package>
+	<package name="${BASEFILENAME}" hash="${HASH}" ${OPTHASH256} size="${SIZE}" required="${REQUIRED}"></package>
 EOF
 done