data: Make expandKeys deterministic
At least in theory, the order the keys are expanded in can make
a difference, particularly if there is key overlap.
We also want to ensure that any underlying base key is processed
before any overridden version of that variable (FOO before FOO_x)
which helps the update_data removal code I've been testing.
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>