VariableDefinedExpr was under-developed, and would not
take into account if a variable was from the globals
or product config dictionary.
It also always emitted `g.get("VARIABLE") != None`, which
is not correct behavior. In this example makefile:
```
MY_VAR :=
ifdef MY_VAR
$(info MY_VAR is defined)
else
$(info MY_VAR is not defined)
endif
ifdef MY_UNDEFINED_VAR
$(info MY_UNDEFINED_VAR is defined)
else
$(info MY_UNDEFINED_VAR is not defined)
endif
MY_VAR ?= true
MY_UNDEFINED_VAR ?= true
$(info MY_VAR after ?= is $(MY_VAR))
$(info MY_UNDEFINED_VAR after ?= is $(MY_UNDEFINED_VAR))
.PHONY: all
all:
@:
```
We get the output:
MY_VAR is not defined
MY_UNDEFINED_VAR is not defined
MY_VAR after ?= is
MY_UNDEFINED_VAR after ?= is true
So we can see that even if a variable was set, it's considered
not defined if it was set to an empty value. However, ?= works
differently, and does require the != None, so that was left
as is.
Just use a variableRefExpr and rely on the fact
that variables will be truthy if they're defined.
Fixes: 216700361
Test: go test
Change-Id: If8944da2579e8658e3fc4f666b1f3b2815f8c8b1
71514c07d2