wpa-supplicant: Use upstream defconfig

wpa-supplicant: Use upstream defconfig

The copy of defconfig we were carrying was from 2014 and very out of
date; drop it and use the upstream version with appropriate edits for
our PACKAGECONFIG.

Switch PACKAGECONFIG to using a default (?=) rather than weak default
(??=) as per current recommendation.

Signed-off-by: Alex Kiernan <alexk@zuma.ai>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>