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

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

Change-Id: I9f02d070454ca931f2f07b7ffd37d1d192be56ca