Rename the variable to make it clearer that it refers to the last Makefile included. Usually this is the current file, but when another Makefile is included this changes.
Test of EXTERNAL_MODULE_DIRS handling
This is a test for the EXTERNAL_MODULE_DIRS variable.
It demonstrates:
- Adding a module with source code
- Setting a header include directory
- Adding dependencies, which are evaluated before other modules dependencies
If the application compiles, everything is ok.