For a project to supply templates, it needs to advertise their location. For this, the tmpl_dir make variable is added to projects.mk. If other-project wants to get hold of some-project's templates, it can do, e.g.:
TEMPLATES = $(wilcard $(call tmpl_dir,some-project)/*.tmpl)
To achieve this, support for the tmpls-dir command is added to jw-projects.py.
Signed-off-by: Jan Lindemann <jan@janware.com>