repack_bootimg: replaces the ramdisk file directly am: d0d261f3b0 am: 0639d39c12 am: d6c8be1c86 am: 21e91e2a7a

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

Change-Id: Id93db925296d32a9d23c15c2f16907a1d4d8eea1