Fix pylint errors going uncaught

Make check-python-files.sh run pylint on all *.py files (in
directories where they are known to be present), rather than list
files explicitly.

Fix a bug whereby the return status of check-python-files.sh was only
based on the last file passing, i.e. errors in other files were
effectively ignored.

Make check-python-files.sh run pylint unconditionally. Since pylint3
is not critical, make all.sh to skip running check-python-files.sh if
pylint3 is not available.
diff --git a/tests/scripts/check-python-files.sh b/tests/scripts/check-python-files.sh
index e64d6b3..9290418 100755
--- a/tests/scripts/check-python-files.sh
+++ b/tests/scripts/check-python-files.sh
@@ -9,10 +9,4 @@
 # Run 'pylint' on Python files for programming errors and helps enforcing
 # PEP8 coding standards.
 
-if `hash pylint3 > /dev/null 2>&1`; then
-    pylint3 -j 2 tests/scripts/generate_test_code.py
-    pylint3 -j 2 tests/scripts/test_generate_test_code.py
-    pylint3 -j 2 tests/scripts/mbedtls_test.py
-else
-    echo "$0: WARNING: 'pylint3' not found! Skipping checks on Python files."
-fi
+pylint3 -j 2 scripts/*.py tests/scripts/*.py