Separate out CASSERT macro into own header

Separate out the CASSERT macro out of bl_common.h into its own
header to allow more efficient header inclusion.

Change-Id: I291be0b6b8f9879645e839a8f0dd1ec9b3db9639
diff --git a/include/bl31/runtime_svc.h b/include/bl31/runtime_svc.h
index 148c0bc..ab2137d 100644
--- a/include/bl31/runtime_svc.h
+++ b/include/bl31/runtime_svc.h
@@ -31,7 +31,7 @@
 #ifndef __RUNTIME_SVC_H__
 #define __RUNTIME_SVC_H__
 #include <psci.h>
-#include <bl_common.h>
+#include <cassert.h>
 
 /*******************************************************************************
  * Bit definitions inside the function id as per the SMC calling convention