Config: Add stm targets in single entry point

This patch allows to group all stm targets in single cmake entry point.
The stm TARGET_PLATFORM should prefix by "STM".
stm platforms supports CoreIPC, CoreIPCTfmLevel2 and RegressionIPCTfmLevel2
Build with GNUARM toolchain.

Change-Id: I6535420b2304c18f2ab260620c38fef632eb7488
Signed-off-by: Ludovic Barre <ludovic.barre@st.com>
Signed-off-by: Michel Jaouen <michel.jaouen@st.com>
diff --git a/platform/ext/stm.cmake b/platform/ext/stm.cmake
new file mode 100644
index 0000000..42aa597
--- /dev/null
+++ b/platform/ext/stm.cmake
@@ -0,0 +1,17 @@
+#-------------------------------------------------------------------------------
+# Copyright (c) 2020, STMicroelectronics - All Rights Reserved
+#
+# SPDX-License-Identifier: BSD-3-Clause
+#
+#-------------------------------------------------------------------------------
+#This file gathers all target of stm.
+
+set(PLATFORM_DIR ${CMAKE_CURRENT_LIST_DIR})
+
+if(${TARGET_PLATFORM} STREQUAL "STM_DISCO_L562QE")
+	include("${PLATFORM_DIR}/target/stm/stm32l5xx/stm32l562e_dk.cmake")
+elseif(${TARGET_PLATFORM} STREQUAL "STM_NUCLEO_L552ZE_Q")
+	include("${PLATFORM_DIR}/target/stm/stm32l5xx/nucleo_l552ze_q.cmake")
+else()
+	message(FATAL_ERROR "ERROR: Target \"${TARGET_PLATFORM}\" is not supported.")
+endif()
\ No newline at end of file