fastboot: copy AVB footer from init_boot.img to end of partition

Similar to GKI boot.img, init_boot.img is released as a specific size
and should be able to be flashed into partitions that have different
sizes. So the AVB footer is moved from the end of the init_boot.img to
the end of the partition in the case that the partition is larger than
the image.

Bug: 203698939
Test: m
Change-Id: Ib5a63a7bf1b4be4eba4941972750f12e3edb3a17