Allow PRODUCT_FULL_TREBLE_OVERRIDE to be false.

Devices which are not Android phones may want to set
PRODUCT_FULL_TREBLE_OVERRIDE (and therefore
PRODUCT_FULL_TREBLE) to false.

Test: confirm expected values when putting in expected values of:
  PRODUCT_FULL_TREBLE_OVERRIDE
  PRODUCT_SHIPPING_API_LEVEL
Fixes: 37869308

Change-Id: If5a383210ea282a3a274e5eba0d32ef6cfebafd0
diff --git a/core/main.mk b/core/main.mk
index 4e9a901..8cb9295 100644
--- a/core/main.mk
+++ b/core/main.mk
@@ -329,8 +329,8 @@
 
 # Boolean variable determining if Treble is fully enabled
 PRODUCT_FULL_TREBLE := false
-ifeq ($(PRODUCT_FULL_TREBLE_OVERRIDE),true)
-  PRODUCT_FULL_TREBLE := true
+ifneq ($(PRODUCT_FULL_TREBLE_OVERRIDE),)
+  PRODUCT_FULL_TREBLE := $(PRODUCT_FULL_TREBLE_OVERRIDE)
 else ifeq ($(PRODUCT_SHIPPING_API_LEVEL),)
   #$(warning no product shipping level defined)
 else ifneq ($(call math_gt_or_eq,$(PRODUCT_SHIPPING_API_LEVEL),26),)