Merge "Allow dtb image to be empty in vendor_boot image" am: eb05d5f2ab am: ef54a13462 am: bb55dd786b am: 42fc3e1d83

Original change: https://android-review.googlesource.com/c/platform/system/tools/mkbootimg/+/2018598

Change-Id: I4ee96d68e7ce1bf230cac50a7f3a0108284d0f6b