To remove redundancy, get-os.sh needs to be retired in favor of
pkg.py distro info. It's needed in platform.mk, but the only
definiton of JW_PKG_PY is in projects.mk, so move it, along with the
variables essential for the command:
include $(JWBDIR)/make/py-version.mk (defining PYTHON)
JW_PKG_PY
DEVELOPMENT
VERSION_FILE
Signed-off-by: Jan Lindemann <jan@janware.com>