Description

When building an application there are several standard directories available in the environment, e.g. BINDIRBASE and so on, which are defined in the root Makefile.include. However, these variables are not available when using a global target such as make doc or make static-test. Some of these would still be useful to have, e.g. see #10038 that introduces a common build directory with the goal to allow for out-of-source-builds in the future, with that in place it would also be valuable to use such common build dir for doc and static-test output, too.

Hence, I would suggest to move all directory variables currently declared into a separate file under makefiles which would allow this file to be included for global targets, too.

Steps to reproduce the issue

use #10038 and run make doc

Expected results

Would be nice to redirect output to a common build directory, like $(RIOTBASE)/build/doc.

Actual results

Doxygen output is always created in $(RIOTBASE)/doc/doxygen/html

Versions

Not important.