python3targetconfig: export _PYTHON_PROJECT_BASE

python3targetconfig: export _PYTHON_PROJECT_BASE

See: https://github.com/python/cpython/commit/2950bc50af8f

As of 3.14.0, Python's sysconfig module looks for _PYTHON_PROJECT_BASE
to be set when cross-compiling, or otherwise it'll find and use the
native interpreter's paths when calling get_path() and filling out
_CONFIG_VARS. Set _PYTHON_PROJECT_BASE to "${B}" in
python3targetconfig's setup_target_config() to help avoid issues with
incorrect paths in rpm and other recipes that depend on this class.

Signed-off-by: Trevor Gamblin <tgamblin@baylibre.com>
Signed-off-by: Antonin Godard <antonin.godard@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>