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

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

Change-Id: Idc50cd0c13e2d7669aab4c0ed5aeb27c5256dfe3