pkgconf: small cleanups

pkgconf: small cleanups

There's no need to --with-pkg-confg-dir as the default is the same and
correct.

Don't append BBCLASSEXTEND, there's no need.

Sync pkg-config-native with the pkg-config recipe so that it sets
the variables in the same way.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>