certify_bootimg: support gki-info.txt

Support reading gki-info.txt from a boot-img.zip, to append
additional settings. e.g., adding BRANCH or BUILD_NUMBER info
into the AVB props of the boot signatures.

Bug: 223288963
Test: atest --host certify_bootimg_test
Change-Id: Ic88602a0d08154e47c0f9f63d946e04ea114d0d4
2 files changed