releasetools: Don't assert an included bootloader

If we are including a bootloader with the update, do not require it
for the OTA since it will be updated with the package.

Change-Id: I074f132925974b1164185294ecfd35e37c6c3972
Merge branch 'cm-11.0' of https://github.com/CyanogenMod/android_device_lge_hammerhead into temp