Make CASSERT() macro callable from anywhere ...

The CASSERT() macro introduces a typedef for the sole purpose of
triggering a compilation error if the condition to check is false.
This typedef is not used afterwards. As a consequence, when the
CASSERT() macro is called from withing a function block, the compiler
complains and outputs the following error message:

  error: typedef 'msg' locally defined but not used [-Werror=unused-local-typedefs]

This patch adds the "unused" attribute for the aforementioned
typedef. This silences the compiler warning and thus makes the
CASSERT() macro callable from within function blocks as well.

Change-Id: Ie36b58fcddae01a21584c48bb6ef43ec85590479

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