Fix non-vendor.img configuration.

If / is not write-able and system.img contains system/vendor, symlink
for `/vendor/ -> /system/vendor/` that is otherwise done in init.rc
should be done at build time.

BUG=b:25512724

Change-Id: Iaa63d6440373a4fd754a933c9f1960b3787a6d98
diff --git a/rootdir/Android.mk b/rootdir/Android.mk
index b134f93..895a25d 100644
--- a/rootdir/Android.mk
+++ b/rootdir/Android.mk
@@ -48,6 +48,8 @@
     ln -sf /storage/self/primary $(TARGET_ROOT_OUT)/sdcard
 ifdef BOARD_VENDORIMAGE_FILE_SYSTEM_TYPE
   LOCAL_POST_INSTALL_CMD += ; mkdir -p $(TARGET_ROOT_OUT)/vendor
+else
+  LOCAL_POST_INSTALL_CMD += ; ln -sf /system/vendor $(TARGET_ROOT_OUT)/vendor
 endif
 ifdef BOARD_ROOT_EXTRA_SYMLINKS
 # BOARD_ROOT_EXTRA_SYMLINKS is a list of <target>:<link_name>.