Add-ons related build system improvements.

Ensure that add-on modules can be embedded, fix up Makefile.moddir_rules
to allow module directory Makefiles to more easily specify the modules to
be built, and explicitly list the addons modules in its Makefile, since
the module names don't follow any pattern.



git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@204415 65c4cc65-6c06-0410-ace0-fbb531ad65f3