Did you check the "external" directory or are you looking for something else? Basically the external directory allows you to symlink an out of tree repository (or repositories) that should contain, at least, the minimal build glues (Makefile and Make.defs). The build system should pick up the rest. The external directory is ignored from the tracking system so nothing impacts the actual apps repo.
The "external" directory is typically a symbolic link. It is mostly of historical interest because you can place any symbolic link to an external directory (or directories) containing a Kconfig and Makefile in the apps/ directory and it (or they) will be built. The only thing special about the "external" directory is that it is properly ignored in apps/.gitignore
