Always generate checksum files on image builds

Checksums are good, lets provide them.

Change-Id: I5d79f3800966dc8e49a892f1097dcbde619ef9a3

ironic_python_agent_builder/__init__.py

@@ -81,6 +81,10 @@ def main():
     if args.lzma:
         os.environ['DIB_IPA_COMPRESS_CMD'] = \
             'xz --format=lzma --compress --stdout'
+    # Enable checksum generation by default
+    if 'DIB_CHECKSUM' not in os.environ:
+        os.environ['DIB_CHECKSUM'] = 'sha256'
+
     extra_args = shlex.split(args.extra_args) if args.extra_args else []
     if args.verbose:
         extra_args.append("-x")