package/pkg-python: use --single-version-externally-managed for host setuptools

package/pkg-python: use --single-version-externally-managed for host setuptools

Currently, host Python packages that use setuptools are installed as
Python Eggs, i.e they are installed in
$(HOST_DIR)/lib/python2.7/site-packages/Something-<version>.egg. Once
installed, each Python Egg is registered to a file called
$(HOST_DIR)/lib/python2.7/site-packages/easy-install.pth. This file is
read by the Python interpreter so that the installation location of
each Egg is added to the Python path, and can be found by the Python
interpreter.

However, the fact that the installation of different Python modules
need to update a common file is clearly not compatible with
per-package directories and top-level parallel build.

To fix this, we avoid using Python Eggs using the same
--single-version-externally-managed option that we use for target
modules. This option is normally meant for distributions packaging
Python modules, and can therefore only be used if either --record (to
record the list of files being installed) or --root is
passed. --root=/ works fine and was suggested by
https://stackoverflow.com/questions/6301003/stopping-setup-py-from-installing-as-egg/33791008#33791008.

With this change, host Python modules installed by setuptools are now
installed in the "regular" way, i.e directly in
$(HOST_DIR)/lib/python2.7/site-packages/mako for host-python-mako.

This makes the installation of host Python modules more similar to the
one of target modules, and makes it compatible with per-package
directory support and top-level parallel build.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Reviewed-by: Asaf Kahlon <asafka7@gmail.com>
Reviewed-by: Yegor Yefremov <yegorslists@googlemail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>