Build: Add isolation level3 config cmakefile

Add ConfigRegressionIPCTfmLevel3.cmake for isolation level3
build and regression test. Only AN521 platform is supported.

Change-Id: I768ae1dd6fe6211f08420a5e52a0961265d61c85
Signed-off-by: Mingyang Sun <mingyang.sun@arm.com>
diff --git a/CommonConfig.cmake b/CommonConfig.cmake
index e8094f5..830c633 100644
--- a/CommonConfig.cmake
+++ b/CommonConfig.cmake
@@ -29,11 +29,7 @@
 #Ignore case on the cmake build types
 string(TOLOWER ${CMAKE_BUILD_TYPE} CMAKE_BUILD_TYPE)
 
-if(CORE_IPC)
-	if (TFM_LVL EQUAL 3)
-		message(FATAL_ERROR "ERROR: Invalid isolation level!")
-	endif()
-else()
+if(NOT CORE_IPC)
 	if(NOT TFM_LVL EQUAL 1)
 		message(FATAL_ERROR "ERROR: Invalid isolation level!")
 	endif()