Add support for CM's bootable/recovery

Credits to Yamil, just edited his previous commit d4e4eb213706e0f05e82bf32857e0d6a1e6af572
a tiny bit to make it universal for all legacy devices

Change-Id: I68721da40369c9f396b84070fa03f3fe5fdf0a5b
releasetools: don't create prebuilt_dir path if it exits

This path may already exists from previous build tasks. Attempting
to create it again will cause an error.

Change-Id: I4baad8bc846470b99f6ce9207c0682401406decc
Merge pull request #6 from AOSPA-legacy/kitkat

merge from AOSPA-legacy