Build: Remove isolation level 3

- Update the comment in config files to indicate that only isolation
  level 1 and 2 are supported.
- Add isolation level check in CommonConfig.cmake.

Change-Id: I0c458b9f0f3f865e1e2659dab05f660389dce306
Signed-off-by: Edison Ai <edison.ai@arm.com>
diff --git a/CommonConfig.cmake b/CommonConfig.cmake
index 1133c97..01c637d 100644
--- a/CommonConfig.cmake
+++ b/CommonConfig.cmake
@@ -19,6 +19,16 @@
 	message(FATAL_ERROR "ERROR: Compiler \"${COMPILER}\" is not supported.")
 endif()
 
+if(CORE_IPC)
+	if (TFM_LVL EQUAL 3)
+		message(FATAL_ERROR "ERROR: Invalid isolation level!")
+	endif()
+else()
+	if(NOT TFM_LVL EQUAL 1)
+		message(FATAL_ERROR "ERROR: Invalid isolation level!")
+	endif()
+endif()
+
 #BL2 bootloader (MCUBoot) related settings
 if(NOT DEFINED BL2)
 	set(BL2 True CACHE BOOL "Configure TF-M to use BL2 and enable building BL2")