Split vendor's system prop overrides into vendor partition

If ENABLE_TREBLE and early mount of vendor partition are enabled,
overrides in PRODUCT_DEFAULT_PROPERTY_OVERRIDES will be stored in
vendor/default.prop.

If ENABLE_TREBLE and vendor partition are enabled, overrides in
PRODUCT_PROPERTY_OVERRIDES will be stored in vendor/build.prop.

default.prop of recovery image will include all the contents of
vendor/default.prop and vendor/build.prop additionally.

ENABLE_EARLY_MOUNT is added to specify whether early mount is enabled
or not, but it should be removed later when early mount becomes
default for all ENABLE_TREBLE products.

Test: building succeeded, and verified on bullhead.
Bug: 34116668
Change-Id: Ic2eb8882ae71921a76c02b3d62d3289c45949295
diff --git a/core/main.mk b/core/main.mk
index 017b818..459e5e7 100644
--- a/core/main.mk
+++ b/core/main.mk
@@ -251,9 +251,15 @@
 ADDITIONAL_BUILD_PROPERTIES += \
   ro.product.first_api_level=$(PRODUCT_SHIPPING_API_LEVEL)
 endif
-ADDITIONAL_BUILD_PROPERTIES := \
-  $(ADDITIONAL_BUILD_PROPERTIES) \
-  $(PRODUCT_PROPERTY_OVERRIDES)
+
+ifneq ($(ENABLE_TREBLE), true)
+  ADDITIONAL_BUILD_PROPERTIES += $(PRODUCT_PROPERTY_OVERRIDES)
+else
+  ifndef BOARD_VENDORIMAGE_FILE_SYSTEM_TYPE
+    ADDITIONAL_BUILD_PROPERTIES += $(PRODUCT_PROPERTY_OVERRIDES)
+  endif
+endif
+
 
 # Bring in standard build system definitions.
 include $(BUILD_SYSTEM)/definitions.mk