vboot: create board-specific test-only GBB HWID if not set

The HWID in vboot GBB is an identifier for machine model. On Chrome OS,
that should be provisioned in manufacturing process (by collecting real
hardware information), and will be checked in system startup.

For bring up developers, they usually prefer to generate a test-only
string for HWID. However that format was not well documented and cause
problems. Further more, most Chromebooks are using HWID v3+ today while
the test-only HWID is usually v2. Non-Chrome OS developers may also
prefer their own format.

To simplify development process, the GBB_CONFIG now defaults to empty
string, and will be replaced by a board-specific test-only v2 HWID
automatically. Developers can still override that in mainboard Kconfig
if they prefer v3 or other arbitrary format.

BUG=b:140067412
TEST=Built 'kukui' successfully. Removed kukui GBB config and built
     again, still seeing correct test HWID.

Change-Id: I0cda17a374641589291ec8dfb1d66c553f7cbf35
Signed-off-by: Hung-Te Lin <hungte@chromium.org>
---
M src/security/vboot/Kconfig
M src/security/vboot/Makefile.inc
M util/chromeos/README.md
A util/chromeos/gen_test_hwid.sh
4 files changed, 58 insertions(+), 1 deletion(-)