repack_bootimg: support legacy boot.img format

Currently repack_bootimg only works for boot.img/vendor_boot.img version
>= 3. This change adds the support for legacy boot.img version 0~2.

Bug: 176789512
Bug: 174443192
Test: atest --host mkbootimg_test
Test: repack_bootimg --src_bootimg boot-debug-4.19-gz.img --dst_bootimg boot_v2.img

Change-Id: Iae3505d66b56f3b3d20f3c4a9915b5ee874b6dc4
3 files changed