classes/pkgconfig: move variable exports to the class

classes/pkgconfig: move variable exports to the class

All recipes that use pkg-config should be inheriting the class so this
is a no-op change for those, and for recipes that do not use pkg-config
will no longer need to rebuild if these variables are changed.

Any recipes that use pkg-config but do not inherit the class will fail
to build, and this is intentional: those recipes should inherit the class.

This commit is simply the move of the exports, the values do not change.

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>