take bootable images from target_files if present
take bootable images from target_files if present

Now that we're building and saving images in the target_files at build
time, we should use those images instead of rebuilding them.

Bug: 17201052
Change-Id: I459e650f66f1e0bdf01ad54df9e34f36bf2ee899

xorware/android_build,xorware/android_build,xorware/android_build,xorware/android_build,xorware/android_build