releasetools: remove boot.img handling

Depends-on: Ic348d31e8df7933dc4f9f055b7a5a2beacc21d32
http://review.cyanogenmod.org/36813

Change-Id: I82b8599f77617e1de3b62cfccc424c139d0cd35e
Merge branch 'cyanogenmod' into cm-10.1
Merge branch 'cm-10.1' into twrp