psa_autogen.py: improve management of output files

While at this, fix also Makefile so that "make clean" does not
complain if some of the files to be cancelled do not exist.

Signed-off-by: Valerio Setti <valerio.setti@nordicsemi.no>
diff --git a/tests/psa-client-server/psasim/Makefile b/tests/psa-client-server/psasim/Makefile
index 396f5ad..db0c412 100644
--- a/tests/psa-client-server/psasim/Makefile
+++ b/tests/psa-client-server/psasim/Makefile
@@ -58,6 +58,5 @@
 	rm -rf libpsaclient libpsaserver
 	rm -rf include/psa_manifest
 	rm -f test/psa_service_* test/psa_notify_*
-	rm -r test/*.log
-	rm test/seedfile
-
+	rm -f test/*.log
+	rm -f test/seedfile