Support 'none' accelerator

Change-Id: Iada7cfaa42dc6f5706504ee78e975730048e0f3c
diff --git a/cmake/helpers.cmake b/cmake/helpers.cmake
index 3444e64..f9c3be0 100644
--- a/cmake/helpers.cmake
+++ b/cmake/helpers.cmake
@@ -160,7 +160,11 @@
 
 function(ethosu_get_architecture config)
     string(TOLOWER ${config} config)
-    if(${config} MATCHES "^ethos-(u[0-9]+|uz)-([0-9]+$)")
+
+    if(${config} STREQUAL "none")
+        set(ETHOSU_ARCH "none" PARENT_SCOPE)
+        set(ETHOSU_NUM_MACS 0 PARENT_SCOPE)
+    elseif(${config} MATCHES "^ethos-(u[0-9]+|uz)-([0-9]+$)")
         set(ETHOSU_ARCH ${CMAKE_MATCH_1} PARENT_SCOPE)
         set(ETHOSU_NUM_MACS ${CMAKE_MATCH_2} PARENT_SCOPE)
     else()