Add 'generate_gki_certificate' standalone tool to certify GKI am: 6ec5817b45 am: c7d21760d2 am: 414d58ed95

Original change: https://android-review.googlesource.com/c/platform/system/tools/mkbootimg/+/1933194

Change-Id: I39e1722aca21d570db8752434f72769e9a95f706