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
(cherry picked from commit c326d24cb9a4ca8af0861f28b67a4f1fe88a69a4)