utils/generate-cyclonedx: fix detecting of tty

Check if the script shall read from stdin if data is piped into.
Otherwise read from the input file or if not specified print usage and
exit.

Signed-off-by: Heiko Thiery <heiko.thiery@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

utils/generate-cyclonedx

@@ -273,13 +273,19 @@ def main():
                 Example usage: make show-info | utils/generate-cyclonedx.py | jq > sbom.json
             '''
         )
-    parser.add_argument("-i", "--in-file", nargs="?", type=argparse.FileType("r"), default=sys.stdin)
-    parser.add_argument("-o", "--out-file", nargs="?", type=argparse.FileType("w"), default=sys.stdout)
+    parser.add_argument("-i", "--in-file", nargs="?", type=argparse.FileType("r"),
+                        default=(None if sys.stdin.isatty() else sys.stdin))
+    parser.add_argument("-o", "--out-file", nargs="?", type=argparse.FileType("w"),
+                        default=sys.stdout)
     parser.add_argument("--virtual", default=False, action='store_true',
                         help="This option includes virtual packages to the CycloneDX output")
 
     args = parser.parse_args()
 
+    if args.in_file is None:
+        parser.print_help()
+        sys.exit(1)
+
     show_info_dict = json.load(args.in_file)
 
     # Remove rootfs and virtual packages if not explicitly included