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 #3 from CyanogenMod/cm-11.0

releasetools: don't create prebuilt_dir path if it exits