Move device specific properties to /vendor

By setting BOARD_PROPERTY_OVERRIDES_SPLIT_ENABLED on aosp_arm64_ab
the device specific properties like ro.product.board
are hosted in /vendor/build.prop.

Test: build and flash aosp_arm64_ab system.img on sailfish and marlin.
Change-Id: I8062ff81221a3026626736b012ceaf99d8ca2d12
diff --git a/target/board/generic_arm64_ab/BoardConfig.mk b/target/board/generic_arm64_ab/BoardConfig.mk
index 6da54dc..8c38ac7 100644
--- a/target/board/generic_arm64_ab/BoardConfig.mk
+++ b/target/board/generic_arm64_ab/BoardConfig.mk
@@ -88,3 +88,5 @@
 # Must be moved after early-mount is supported.
 ###############################################################################
 BOARD_SEPOLICY_DIRS += device/google/marlin/sepolicy
+
+BOARD_PROPERTY_OVERRIDES_SPLIT_ENABLED := true