python3targetconfig.bbclass: Extend PYTHONPATH instead of overwriting

python3targetconfig.bbclass: Extend PYTHONPATH instead of overwriting

[YOCTO #15108] -- https://bugzilla.yoctoproject.org/show_bug.cgi?id=15108

Since the latest change, the PYTHONPATH is overwritten instead of extended.
This leads to changed behavior and build errors of recipes where the PYTHONPATH
is set before setup_target_config is run.

Fixes: c9617c03bcee ("python3targetconfig.bbclass: use PYTHONPATH to point to the target config")
Signed-off-by: Johannes Schrimpf <dev@loewen-email.de>
[Luca: add Fixes: tag]
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>