repack_bootimg: support vendor_boot v4 am: 7e76cba96d am: e4c3a59b97 am: 3f40ab1afc am: c701df651f am: 1aef2adc8e am: fe591437ed

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

Change-Id: I32e484143d938f71849cd5879154d99530e13137