Merge change 2951 into donut

* changes:
  support incremental updates of boot image
tree: b68657475aaa2e29da2561a828b5db3ca8c4d06a
  1. core/
  2. libs/
  3. target/
  4. tools/
  5. buildspec.mk.default
  6. cleanspec.mk
  7. envsetup.sh