vboot2: Clean up signing data for unit tests

Signing is now simple enough (thanks to full utility lib support for
vboot2 data structs) that we don't need the vb2_convert_structs module
anymore.  Also, use the utility lib function to create a firmware
preamble, rather than duplicating that code in the fwlib unit tests.

BUG=chromium:423882
BRANCH=none
TEST=VBOOT2=1 make runtests, and build firmware for veyron_pinky

Change-Id: I1db402a08621f79274d2a69095aebc3e84f4328d
Signed-off-by: Randall Spangler <rspangler@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/232755
6 files changed