repack_bootimg: allow specifying the copy-to file path am: d7eda242c4 am: 0b2e3a3f5b am: c13fa191a7

Original change: https://android-review.googlesource.com/c/platform/system/tools/mkbootimg/+/1679427

Change-Id: I853e84939f5985c35102f436be5877cc1c06fcee