Updating makefile to document key generation

Signed-off-by: Mukesh Bharsakle <bharsaklemukesh975@gmail.com>
diff --git a/tests/data_files/Makefile b/tests/data_files/Makefile
index 4228f45..c23db51 100644
--- a/tests/data_files/Makefile
+++ b/tests/data_files/Makefile
@@ -41,6 +41,10 @@
 test_ca_pwd_rsa = PolarSSLTest
 test_ca_config_file = test-ca.opensslconf
 
+$(test_ca_key_file_rsa):$(test_ca_pwd_rsa)
+    $(OPENSSL) genrsa -aes-128-cbc -passout pass:$< -out $@ 2048
+all_final += $(test_ca_key_file_rsa)
+
 test-ca.req.sha256: $(test_ca_key_file_rsa)
 	$(MBEDTLS_CERT_REQ) output_file=$@ filename=$(test_ca_key_file_rsa) password=$(test_ca_pwd_rsa) subject_name="C=NL,O=PolarSSL,CN=PolarSSL Test CA" md=SHA256
 all_intermediate += test-ca.req.sha256