Build: A switch for selecting platform service

Switch 'TFM_PARTITION_PLATFORM' is added for selecting platform
service. Enabled by default and can be overridden by cmake command.

Change-Id: Ic2529b101292b8f81203da4a11815778194806f9
Signed-off-by: Mingyang Sun <mingyang.sun@arm.com>
diff --git a/secure_fw/ns_callable/CMakeLists.inc b/secure_fw/ns_callable/CMakeLists.inc
index 91f803a..5664a8e 100644
--- a/secure_fw/ns_callable/CMakeLists.inc
+++ b/secure_fw/ns_callable/CMakeLists.inc
@@ -23,8 +23,11 @@
 	message(FATAL_ERROR "Please set TFM_ROOT_DIR before including this file.")
 endif()
 
-set (SS_NS_CALLABLE_C_SRC "${CMAKE_CURRENT_LIST_DIR}/tfm_veneers.c"
-                          "${CMAKE_CURRENT_LIST_DIR}/tfm_platform_veneers.c")
+set (SS_NS_CALLABLE_C_SRC "${CMAKE_CURRENT_LIST_DIR}/tfm_veneers.c")
+
+if (TFM_PARTITION_PLATFORM)
+    list(APPEND SS_NS_CALLABLE_C_SRC "${CMAKE_CURRENT_LIST_DIR}/tfm_platform_veneers.c")
+endif()
 
 if (TFM_PARTITION_AUDIT_LOG)
 	list(APPEND SS_NS_CALLABLE_C_SRC "${CMAKE_CURRENT_LIST_DIR}/tfm_audit_veneers.c")