repack_bootimg: replaces the ramdisk file directly am: d0d261f3b0 am: 0639d39c12 am: d6c8be1c86

Original change: https://android-review.googlesource.com/c/platform/system/tools/mkbootimg/+/1696925

Change-Id: I114fbba6fbdd8693235b336d45155216128d2fd8