don't build libbt-vendor when vendor/ is missing
am: d87c2ee912

Change-Id: I8f0aa05aaf4bb29764e7644db0ae526411648db5
tree: f696a0dc2115747413b9be648bd67d9123b1d2d1
  1. libbt-vendor/
  2. Android.mk
  3. CleanSpec.mk