Fix single value inheritance

Single-value product variables resolve to the first
instance of that variable in prefix order. What we
have right now is taking the value from the current
makefile if it's set, but if it's not, we take it
from the last child, not the first.

Fixes: 226206409
Test: Manually
Change-Id: Id23f7c269ff9a352bf0b67cb57156b72449f978e