Don't just assign but append to EXTRA_OECONF so we maintain python-dir
setting fo EXTRA_OECONF.
This fixes issue if libdir is set to something like '/usr/lib64'.
(From OE-Core rev: a92d56058b21913570bb17ae416c3b00afce055e)
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>