sign_official_build: Support new image layout (always installable kernel in B).
sign_official_build: Support new image layout (always installable kernel in B).

ChromiumOS images have recently changed the installable vblock from stateful
partition to kernel + vblock in slot B. sign_official_build script should
follow that layout so other scripts (ex, cros_generate_update_payload) won't
find wrong blob.

BRANCH=none
BUG=chrome-os-partner:32156
TEST=for image_t in ssd usb install recovery; do
       ./sign_official_build.sh $image_t IMAGE KEYDIR output$image_t.bin
       ./sign_official_build.sh verify output$image_t.bin
     done
     # Also boots images without problem.

Change-Id: I04e2b50f3f3355263ba6de9567b4a82c040c5826
Reviewed-on: https://chromium-review.googlesource.com/221890
Reviewed-by: Hung-Te Lin <8a7fce4940baad93a69f5073b383c85deed9d9c4@chromium.org>
Tested-by: Hung-Te Lin <8a7fce4940baad93a69f5073b383c85deed9d9c4@chromium.org>
Commit-Queue: Hung-Te Lin <8a7fce4940baad93a69f5073b383c85deed9d9c4@chromium.org>

coreboot/vboot,coreboot/vboot,acorn-marvell/vboot_reference,geekboxzone/mmallow_external_vboot_reference,geekboxzone/mmallow_external_vboot_reference,coreboot/vboot,coreboot/vboot,geekboxzone/mmallow_external_vboot_reference,acorn-marvell/vboot_reference,geekboxzone/mmallow_external_vboot_reference,acorn-marvell/vboot_reference,coreboot/vboot,acorn-marvell/vboot_reference,geekboxzone/mmallow_external_vboot_reference