This installs python{2.7,3.7}-config in $(STAGING_DIR)/usr/bin as part
of Build/InstallDev, to be used by other packages to get build
configuration for target Python.

The treatment for Python 2 and 3 are a bit different:

* For Python 2, python-config is a Python script that is expected to be
  run with, and return data for, the installed Python interpreter. This
  installs a modified version of this script, to be run using host
  Python, and read/return data for target Python.

* Python 3 includes a shell script version of python-config (expected to
  be used in cross-compilation scenarios). This simply installs the
  script into the right place.

Signed-off-by: Jeffery To <jeffery.to@gmail.com>