am def30fc2: Clarify the use of --boot-dirs in the header comment.

Merge commit 'def30fc213d62fc93edce2ca63967ab93380075d' into gingerbread-plus-aosp

* commit 'def30fc213d62fc93edce2ca63967ab93380075d':
  Clarify the use of --boot-dirs in the header comment.