IVGCVSW-4915 Improving messages around SAMPLE_DYNAMIC_BACKEND_ENABLED

* If SAMPLE_DYNAMIC_BACKEND_ENABLED is enabled but DYNAMIC_BACKEND_PATHS
  is not set then the user gets failing tests cases with no clear idea
  what has gone wrong. Explicitly failing when DYNAMIC_BACKEND_PATHS has
  not been set.
* Adding an FAQ entry describing how the user can get confused by our
  documented usage of CMake.

Signed-off-by: Colm Donelan <Colm.Donelan@arm.com>
Change-Id: Ic11e14a971f53a6454962354ff728365ccc9de6d
diff --git a/cmake/GlobalConfig.cmake b/cmake/GlobalConfig.cmake
index 6aa6321..650d409 100644
--- a/cmake/GlobalConfig.cmake
+++ b/cmake/GlobalConfig.cmake
@@ -26,6 +26,7 @@
 option(TF_LITE_GENERATED_PATH "Tensorflow lite generated C++ schema location" OFF)
 option(FLATBUFFERS_ROOT "Location where the flatbuffers 'include' and 'lib' folders to be found" Off)
 option(DYNAMIC_BACKEND_PATHS "Colon seperated list of paths where to load the dynamic backends from" "")
+option(SAMPLE_DYNAMIC_BACKEND "Include the sample dynamic backend and its tests in the build" OFF)
 option(BUILD_GATORD_MOCK "Build the Gatord simulator for external profiling testing." ON)
 option(BUILD_TIMELINE_DECODER "Build the Timeline Decoder for external profiling." ON)
 option(SHARED_BOOST "Use dynamic linking for boost libraries" OFF)