Use BOARD_USES_VENDORIMAGE to detect vendor image presence

When building without vendor/... in the tree, we won't be creating a
vendorimage, so BOARD_VENDORIMAGE_FILE_SYSTEM_TYPE won't be defined. But
we still need to use the vendor image that will be present.

Bug: 30040825
Change-Id: I756adc615aea11da60da4d3a4568caca9fed93ab
diff --git a/rootdir/Android.mk b/rootdir/Android.mk
index d53af2f..e060a2c 100644
--- a/rootdir/Android.mk
+++ b/rootdir/Android.mk
@@ -58,7 +58,7 @@
     ln -sf /system/etc $(TARGET_ROOT_OUT)/etc; \
     ln -sf /sys/kernel/debug $(TARGET_ROOT_OUT)/d; \
     ln -sf /storage/self/primary $(TARGET_ROOT_OUT)/sdcard
-ifdef BOARD_VENDORIMAGE_FILE_SYSTEM_TYPE
+ifdef BOARD_USES_VENDORIMAGE
   LOCAL_POST_INSTALL_CMD += ; mkdir -p $(TARGET_ROOT_OUT)/vendor
 else
   LOCAL_POST_INSTALL_CMD += ; ln -sf /system/vendor $(TARGET_ROOT_OUT)/vendor