package/pkg-python: use proper --prefix and --root values

package/pkg-python: use proper --prefix and --root values

Some python software refers to setuptool/distutils options
to install files in python root directory (like data_files option).
To use this type of option, python root should point to the real python
root in buildroot folder and not to the guest os /.
Prefix path is always built starting from the python root, so it should
be simply /usr.

Signed-off-by: Angelo Compagnucci <angelo@amarulasolutions.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>