Some packages (PyYAML,
https://github.com/openwrt/packages/pull/8482#discussion_r270692276)
recognize "global" options to setup.py; these must appear before the
"install" command on the command line.
This adds PYTHON[3]_PKG_SETUP_GLOBAL_ARGS, which let packages set these
global options.
Signed-off-by: Jeffery To <jeffery.to@gmail.com>