fastboot: copy AVB footer on boot image to end of partition

If the flashed boot image is smaller than the block device, the AVB
footer will not be at the end of the partition. Although images are
normally created to match the partition size the GKI boot.img must work
on all devices, and the size of the boot partition will vary.

Copy the AVB footer to the end of the partition before flashing, if it
is not there already.

Bug: 159377163
Change-Id: I5a5e25fb54dc9d6a2930fda63434968808ffa1f0