repack_bootimg: allow specifying the copy-to file path am: d7eda242c4 am: 0b2e3a3f5b am: c13fa191a7 am: 8e716b4e5c am: 7fa831b424

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

Change-Id: Ib5c9c3303de6810ad54ab27b189a86ac7b36f6e8