Split venv setup and package install targets

This allow to reinstall the packages without
rebuilding the venv in case of dependencies change.

tools/python.mk

@@ -20,6 +20,10 @@ SHARED_DEV_REQUIREMENTS = \
 VENV = venv
 $(VENV):
 	python3 -m venv $(VENV)
+	source $(VENV)/bin/activate
+	$(MAKE) install
+
+install: venv
 	source $(VENV)/bin/activate
 	pip install --upgrade pip setuptools wheel
 	pip install $(SHARED_DEV_REQUIREMENTS) $(PIP_INSTALL)