Build: Use the TARGET_PLATFORM_PATH

Change-Id: I4b33600ca6ccdaff60994a498123107054eaa62e
Signed-off-by: Kevin Peng <kevin.peng@arm.com>
diff --git a/app/CMakeLists.txt b/app/CMakeLists.txt
index c0146a4..97fd3c5 100755
--- a/app/CMakeLists.txt
+++ b/app/CMakeLists.txt
@@ -14,8 +14,8 @@
 # Secure code. To facilitate this, we once again reload the compiler to load the
 # setting for the NS CPU. Cmake settings are directory scoped so this affects
 # anything loaded from or declared in this dir.
-if (EXISTS ${CMAKE_SOURCE_DIR}/platform/ext/target/${TFM_PLATFORM}/preload_ns.cmake)
-    include(${CMAKE_SOURCE_DIR}/platform/ext/target/${TFM_PLATFORM}/preload_ns.cmake)
+if (EXISTS ${TARGET_PLATFORM_PATH}/preload_ns.cmake)
+    include(${TARGET_PLATFORM_PATH}/preload_ns.cmake)
     tfm_toolchain_reload_compiler()
 endif()