make: introduce common Python lib path

Introduce dist/pythonlibs directory to store RIOT python packages.
This directory is exported via PYTHONPATH by the build system to
make it commonly available.

Makefile.include

@@ -70,6 +70,9 @@ BUILDRELPATH ?= $(CURDIR:$(RIOTPROJECT)/%=%)/
 # get host operating system
 OS := $(shell uname)
 
+# set python path, e.g. for tests
+PYTHONPATH := $(RIOTBASE)/dist/pythonlibs/:$(PYTHONPATH)
+
 # Include Docker settings near the top because we need to build the environment
 # command line before some of the variable origins are overwritten below when
 # using abspath, strip etc.

dist/pythonlibs/README.md

+This directory is exported through PYTHONPATH environment variable in the build system.
+Put any RIOT specific Python packages here.

makefiles/vars.inc.mk

@@ -31,6 +31,8 @@ export BINDIR                # This is the folder where the application should b
 export APPDIR                # The base folder containing the application
 export PKGDIRBASE            # The base folder for building packages
 
+export PYTHONPATH            # Python default search path for module filesi, with RIOT specific packages
+
 export FEATURES_REQUIRED     # List of required features by the application
 export FEATURES_PROVIDED     # List of provided features by the board
 export FEATURES_OPTIONAL     # List of nice to have features