Add 'retrofit_gki' tool that retrofits GKIs am: a37920c9be am: b11e6f8d1c am: 18ea7b1fa4

Original change: https://android-review.googlesource.com/c/platform/system/tools/mkbootimg/+/1952245

Change-Id: Ibcd955ed516158ba011ba16da87f4f5763f805e2