cmake: Check for valid voerride values

Also remove trailing whitespace

cmake/functions.cmake

@@ -1,4 +1,11 @@
 function(override_cache VAR VAL)
+    get_property(VAR_STRINGS CACHE ${VAR} PROPERTY STRINGS)
+    LIST(FIND VAR_STRINGS ${VAL} CK)
+    if(-1 EQUAL ${CK})
+        message(SEND_ERROR
+            "\"${VAL}\" is not valid override value for \"${VAR}\"."
+            " Please select value from \"${VAR_STRINGS}\"\n")
+    endif()
     set_property(CACHE ${VAR} PROPERTY VALUE ${VAL})
 endfunction()