vboot: merge vboot_api_stub_init.c into vboot2 namespace

Move vb2ex_mtime stub from vboot_api_stub_init
into 2lib/2stub.c in vboot2 namespace.

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

Signed-off-by: Joel Kitching <kitching@google.com>
Change-Id: Ica27630090f854e1abc56acae0294b68deb74a0f
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/vboot_reference/+/2635676
Reviewed-by: Joel Kitching <kitching@chromium.org>
Tested-by: Joel Kitching <kitching@chromium.org>
Commit-Queue: Joel Kitching <kitching@chromium.org>
4 files changed