Adapt build to using cmsis_system

The core_software handling has split cmsis_startup into cmsis_system and
cmsis_startup in order to be able to controll the build of the libraries
per excecutable. This change adapts to that change.

Change-Id: I13286c75ba1c2298e760f9c0594ce4fec9db33dc
diff --git a/targets/corstone-300/CMakeLists.txt b/targets/corstone-300/CMakeLists.txt
index d162537..03226c7 100644
--- a/targets/corstone-300/CMakeLists.txt
+++ b/targets/corstone-300/CMakeLists.txt
@@ -50,6 +50,9 @@
 # Corstone-300
 #############################################################################
 
+# Enable trustzone support in core_software
+set(TRUSTZONE_BUILD ON)
+
 add_subdirectory(${CMAKE_CURRENT_SOURCE_DIR}/../common target)
 
 # Linker script