vboot: collect UI-related stubs in vboot2 namespace

Collect all UI-related stubs in the vboot2 namespace
within 2lib/2stub.c.

BUG=b:124141368, chromium:968464
TEST=make clean && make runtests
BRANCH=none

Signed-off-by: Joel Kitching <kitching@google.com>
Change-Id: I64cb44f5e1db25b96a1a4a266b78f02e82110c5f
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/vboot_reference/+/2635674
Tested-by: Joel Kitching <kitching@chromium.org>
Commit-Queue: Joel Kitching <kitching@chromium.org>
Reviewed-by: Yu-Ping Wu <yupingso@chromium.org>
3 files changed