unpack_bootimg: Export 'vendor_ramdisk' for vendor_boot v4 am: 33bf962400 am: 9d1f71ae36 am: d4ad80ee44

Original change: https://android-review.googlesource.com/c/platform/system/tools/mkbootimg/+/1952244

Change-Id: I9776168bdfda14a27063394b536c319a202be1eb