Buildsystem restructuring

- Allow linker file to be overriden
- Disable Trustzone build for other targets than Corstone-300
- Make toolchain TARGET_CPU variable cached

Change-Id: I98a15e1080e4bf49e029578888b1e4ce362bbab7
diff --git a/applications/trustzone_inference/CMakeLists.txt b/applications/trustzone_inference/CMakeLists.txt
index f23c7f6..f1efa95 100644
--- a/applications/trustzone_inference/CMakeLists.txt
+++ b/applications/trustzone_inference/CMakeLists.txt
@@ -16,6 +16,10 @@
 # limitations under the License.
 #
 
+if (NOT ${ETHOSU_TARGET} STREQUAL "corstone-300")
+    return()
+endif()
+
 #############################################################################
 # Secure world
 #############################################################################