As the code stands, setting a variable with ??= could result in a ?=
variable not overriding it. This patch fixes the issue by allowing
the ast to make lookups that ignore any ??= set variables.
(Bitbake rev: 32fee2e650dfdd3aa9a7572dad1251e0c24ca34b)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>