Test: Add fetch eRPC remote library

Add fetch eRPC remote library.
eRPC is integrated with tf-m tests.

Signed-off-by: Summer Qin <summer.qin@arm.com>
Change-Id: Iad79d5169e3345b52deb79ea2855c29453297db2
diff --git a/lib/ext/CMakeLists.txt b/lib/ext/CMakeLists.txt
new file mode 100644
index 0000000..eb53c97
--- /dev/null
+++ b/lib/ext/CMakeLists.txt
@@ -0,0 +1,10 @@
+#-------------------------------------------------------------------------------
+# Copyright (c) 2023, Arm Limited. All rights reserved.
+#
+# SPDX-License-Identifier: BSD-3-Clause
+#
+#-------------------------------------------------------------------------------
+
+if(CONFIG_TFM_ERPC_TEST_FRAMEWORK)
+    add_subdirectory(erpc)
+endif()