vboot: fold lib20/api.c into 2lib/2api.c

All of the functions in lib20/api.c are actively used in coreboot
verstage firmware verification.  Fold the functions and their
test suite into 2lib.

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

Change-Id: I2dbad2715246a8a16ee85dac553a751ae1590afa
Signed-off-by: Joel Kitching <kitching@google.com>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/vboot_reference/+/1659991
Tested-by: Joel Kitching <kitching@chromium.org>
Commit-Queue: Joel Kitching <kitching@chromium.org>
Reviewed-by: Julius Werner <jwerner@chromium.org>
5 files changed