Building without NPU

Support building applications for targets without a NPU.

Change-Id: I04199b1c0444f23eadc6d4cf85195413dfd8bc97
diff --git a/targets/corstone-300/CMakeLists.txt b/targets/corstone-300/CMakeLists.txt
index 7ae4d04..9c527fc 100644
--- a/targets/corstone-300/CMakeLists.txt
+++ b/targets/corstone-300/CMakeLists.txt
@@ -76,8 +76,13 @@
     target.cpp
     mpu.cpp)
 
-target_compile_definitions(ethosu_core_driver PUBLIC ETHOSU)
-target_link_libraries(ethosu_target_startup INTERFACE ethosu_core_driver timing_adapter)
+target_link_libraries(ethosu_target_startup INTERFACE
+    $<$<TARGET_EXISTS:ethosu_core_driver>:ethosu_core_driver;timing_adapter>)
+
+if (TARGET ethosu_core_driver)
+    target_compile_definitions(ethosu_core_driver PUBLIC
+        ETHOSU)
+endif()
 
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 # Applications