python3: fix patching get_python_lib() in distutils/sysconfig.py

python3: fix patching get_python_lib() in distutils/sysconfig.py

Previous, two things were wrong:
1) lib_basename was set from STAGING_LIBDIR only if prefix parameter was empty or missing
2) if prefix was not empty, lib_basename reverted to sys.lib, even if STAGING_LIBDIR
should've overriden it

Signed-off-by: Alexander Kanavin <alexander.kanavin@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>