Exclude applications that depend on NPU

Exclude applications that depend on the NPU, if the NPU driver
library was not compiled by the project.

Change-Id: I3fcb570cf155d589a9576266332ba519a11186f1
diff --git a/applications/driver_unit_tests/CMakeLists.txt b/applications/driver_unit_tests/CMakeLists.txt
index 45c733b..aee3025 100644
--- a/applications/driver_unit_tests/CMakeLists.txt
+++ b/applications/driver_unit_tests/CMakeLists.txt
@@ -16,6 +16,10 @@
 # limitations under the License.
 #
 
+if (NOT TARGET ethosu_core_driver)
+  return()
+endif()
+
 add_library(driver_unit INTERFACE)
 
 target_sources(driver_unit INTERFACE