releasetools: use prebuilt boot image if using a custom boot image makefile

Change-Id: Ic348d31e8df7933dc4f9f055b7a5a2beacc21d32
Merge "releasetools: use prebuilt boot image if using a custom boot image makefile" into jb4.3
Merge pull request #5 from SlimRoms/jb4.3

merge latest slim