repack_bootimg: support vendor_boot v4 am: 7e76cba96d am: e4c3a59b97 am: 3f40ab1afc

Original change: https://android-review.googlesource.com/c/platform/system/tools/mkbootimg/+/1695291

Change-Id: I89b2e9601e54ec3a916f2c3604607121a2a2662d