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

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

Change-Id: Ia3b58c2ddc93ed49fbd1df2fe61ca5492ba0b96c