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/context.h b/include/bl31/context.h
index cb4cd8e..b0c9810 100644
--- a/include/bl31/context.h
+++ b/include/bl31/context.h
@@ -31,8 +31,8 @@
 #ifndef __CONTEXT_H__
 #define __CONTEXT_H__
 
-#include <bl_common.h>
 #include <arch.h>
+#include <cassert.h>
 
 /*******************************************************************************
  * Constants that allow assembler code to access members of and the 'gp_regs'