uninative needs to adjust NATIVELSBSTRING fairly late in the
configuration parsing process but the sstate code encodes it into
variables. Since this string doesn't vary on a per recipe basis, we
defer its expansion until usage time.
(From OE-Core rev: fb680ab8b3cf7aba9b579403b1aeb96d30762320)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>