LAKITU: generate_update_payload: fix exit code

GenerateUpdatePayload returns True on success, and False on failure.
However, cros_generate_update_payload returns this value in main(). The
chromite wrapper script runs sys.exit() with this value, and
sys.exit(True) results in an exit code of 1, which is usually
interpreted as a failure. To make the exit code consistent with usual
conventions, invert the value when we return from main().

BUG=b/257271340
TEST=presubmit
RELEASE_NOTE=None

Change-Id: I6285b861a77c9186aeaecf79d4708f00900573f1
diff --git a/scripts/cros_generate_update_payload.py b/scripts/cros_generate_update_payload.py
index d8aea88..580adcc 100644
--- a/scripts/cros_generate_update_payload.py
+++ b/scripts/cros_generate_update_payload.py
@@ -59,7 +59,7 @@
                                                            opts.output)
     return
 
-  return paygen_payload_lib.GenerateUpdatePayload(
+  return not paygen_payload_lib.GenerateUpdatePayload(
       opts.tgt_image, opts.output, src_image=opts.src_image,
       work_dir=opts.work_dir, private_key=opts.private_key, check=opts.check,
       minios=opts.minios)