Build: Use CMAKE_SOURCE_DIR to set default TFM_ROOT_DIR

The default value of TFM_ROOT_DIR is hard-coded to
trusted-firmware-m. But not everyone is using exactly the
same name.
This patch set the default value to CMAKE_SOURCE_DIR, which is
set by CMake command: cmake [options] <path-to-source>, so that
the dependency on the repo name is removed.

Change-Id: I6d1a8028e34cc07e20be8e7671812b2fde6feede
Signed-off-by: Kevin Peng <kevin.peng@arm.com>
diff --git a/test/CMakeLists.txt b/test/CMakeLists.txt
index 2c5ea2d..969a941 100644
--- a/test/CMakeLists.txt
+++ b/test/CMakeLists.txt
@@ -8,7 +8,7 @@
 cmake_minimum_required(VERSION 3.7)
 
 if (NOT DEFINED TFM_ROOT_DIR)
-	set(TFM_ROOT_DIR ${CMAKE_CURRENT_LIST_DIR}/../../trusted-firmware-m)
+	set(TFM_ROOT_DIR ${CMAKE_SOURCE_DIR})
 endif()
 
 #Tell cmake where our modules can be found