Remove ?= assignements to product variables

In make, all product variables are assigned to an empty string before
including the product config makefiles. In starlark, we don't do that
just so the memory usage is smaller. This means that in make, using
?= to assign a product config variable has no effect. Replicate this
behavior in starlark by not emitting any code for ?= assignments of
product variables.

Fixes: 304324003
Test: go test
Change-Id: Id31531506ac9e372a1bea92ce9ae8e17ae0ca70c