Update build script for tests after mbedcrypto exporter

Update to the latest syntax changes of generate_test_code.py. This was
missed in the rebase onto mbedtls-2.13.
diff --git a/crypto/tests/Makefile b/crypto/tests/Makefile
index 2de5ffa..f76c1c0 100644
--- a/crypto/tests/Makefile
+++ b/crypto/tests/Makefile
@@ -41,7 +41,7 @@
 		-t suites/main_test.function \
 		-p suites/host_test.function \
 		-s suites  \
-		--help-file suites/helpers.function \
+		--helpers-file suites/helpers.function \
 		-o .
 
 
@@ -70,7 +70,7 @@
 		-t suites/main_test.function \
 		-p suites/target_test.function \
 		-s suites  \
-		--help-file suites/helpers.function \
+		--helpers-file suites/helpers.function \
 		-o ./TESTS/mbedcrypto/$*
 
 gen-embedded-test: $(EMBEDDED_TESTS)