build(tools/cbmc): integrate cbmc-viewer

Add option to the build system to call cbmc-viewer. The following
changes were necessary:

 - Add option to make call of cbmc-viewer optional
 - Update CBMC Application note with cbmc-viewer
 - Separate CBMC run to 2 steps: compiling to GOTO program, and call
   CBMC on the resulting goto program. This is necessary becasue
   cbmc-viewer operates on the GOTO program.
 - Rewrite extraction of summary, as XML output (required by CBMC) files
   doesn't contain summary

Change-Id: Ieb2623d3b03845dcb4ab74baead81d3c0fc6dee8
Signed-off-by: Mate Toth-Pal <mate.toth-pal@arm.com>