Added the possibility to log NAS messages in UE (ENABLE_NAS_UE_LOGGING must be added in the build file and "opennair-cn" directory must be available to access NAS header files).

git-svn-id: http://svn.eurecom.fr/openair4G/trunk@4975 818b1a75-f10b-46b9-bf7c-635c3b92a50f