Add variables to support checking system manifest.

System manifest is determined when all of /system/etc/vintf/manifest*
are built. Export a few variables to support checking
system manifest against frozen data.

Bug: 37226359
Test: freeze.sh 5 && m check-vintf-all

Change-Id: I9906e888e3cb7d14851a98526adf02706c4ed08b
diff --git a/vintfdata/Android.mk b/vintfdata/Android.mk
index d873e29..a7721c2 100644
--- a/vintfdata/Android.mk
+++ b/vintfdata/Android.mk
@@ -128,3 +128,5 @@
 SYSTEM_EXT_MANIFEST_INPUT_FILES :=
 DEVICE_MATRIX_INPUT_FILE :=
 PRODUCT_MANIFEST_INPUT_FILES :=
+
+VINTF_FRAMEWORK_MANIFEST_FROZEN_DIR := $(LOCAL_PATH)/frozen