depthcharge: replace gbb_get_flags with vb2api_gbb_get_flags

Remove gbb_get_flags function and replace with vboot API
vb2api_gbb_get_flags.

BUG=b:124141368, chromium:954774
TEST=Build locally
BRANCH=none

Change-Id: I87ca20904045f5664ef361215248a2d8cf26068f
Signed-off-by: Joel Kitching <kitching@google.com>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/depthcharge/+/1663748
Tested-by: Joel Kitching <kitching@chromium.org>
Auto-Submit: Joel Kitching <kitching@chromium.org>
Reviewed-by: Furquan Shaikh <furquan@chromium.org>
Reviewed-by: Julius Werner <jwerner@chromium.org>
Commit-Queue: Joel Kitching <kitching@chromium.org>
5 files changed