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
1 file changed