base.bbclass: fix nondeterministic PACKAGECONFIG processing order

The PACKAGECONFIG flags were iterated over using dict.items(), but this
returns the items in an undefined order. As this order determines the
EXTRA_OECONF append order, we can get EXTRA_OECONF which are functionally
equivalent, but whose contents differ, resulting in not using shared state
archives we should be using.

Signed-off-by: Christopher Larson <kergoth@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>