Actually fail if `--check` fails

Return a nonzero status if called with `--check` and the check finds some
problems.

Fixes #244.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>

scripts/make_generated_files.py

diff --git a/scripts/make_generated_files.py b/scripts/make_generated_files.py
index 176ccd8..d919fb0 100755
--- a/scripts/make_generated_files.py
+++ b/scripts/make_generated_files.py
@@ -242,6 +242,7 @@
                 # there's nothing to compare to, or clean up.
                 continue
             if not filecmp.cmp(file, bak_file):
+                ok = False
                 ref_file = file.with_name(file.name + ".ref")
                 ref_file = root / ref_file
                 if ref_file.exists():