Makefile.include: fix override headers from application

[leandrolanzieri]

Contribution description

As our drivers guide states:

The idea is that this file can be overridden by an application or a board, by simply putting a file with the same name in the application's or the board's include folders, while RIOT's build system takes care of preferring those files instead of the default params file.

For some reason this doesn't seem to be working. I'm not sure whether there was some uncaught regression at some point. Currently, when one places a <driver>_params.h this doesn't override the one define by RIOT, and is simply ignored.

This PR fixes the issue by including the APPDIR in the considered include paths, so that it has priority.

Testing procedure

• On some driver test application, attempt to override default parameters by defining a <driver>_params.h file in your app directory.

Issues/PRs references

None