fastboot: Copy AVB footer from recovery.img to the end of the partition

Copy the AVB footer of the recovery.img to the end of the recovery
partition, so that it can be found when loading a recovery image
from a partition that is larger than the image.

Bug: 261673286
Test: m and flashing recovery image and bootloader can now detect
AVB footer.
Change-Id: Ib93da33c72f3a96eb8d9bcedaab5ef0c3a4d418d
Signed-off-by: Isaac J. Manjarres <isaacmanjarres@google.com>