vboot: implement vb2api_get_recovery_reason

This API function should be used rather than directly
accessing vb2_shared_data.recovery_reason.

BUG=b:124141368, chromium:1038260, chromium:957880
TEST=make clean && make runtests
BRANCH=none

Change-Id: I92c8f9a654400be69885a691b39c4b9e4e2031c1
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/vboot_reference/+/2055662
Reviewed-by: Julius Werner <jwerner@chromium.org>
Commit-Queue: Joel Kitching <kitching@chromium.org>
Tested-by: Joel Kitching <kitching@chromium.org>
3 files changed