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: Ie6d42388b13bff044fe63a9925e32f680d37c48e