fw_config: Use UNDEFINED_FW_CONFIG to mean unprovisioned

A mainboard might want to configure some things differently when a
device is in an unprovisioned state. In the case when fw_config comes
from the Chromium EC, an unprovisioned device will not have a FW_CONFIG
tag in its CBI. This patch will set the fw_config value to
UNDEFINED_FW_CONFIG in the case of an error retrieving the value, as
well as adding a function, `fw_config_is_provisioned()` to indicate the
provisioning status.

BUG=none
TEST=remove fw_config from chromium EC CBI, add code to mainboard to
print return value of fw_config_is_provisioned() (`0`), add
fw_config back to CBI, run same test and see `1`.

Signed-off-by: Tim Wawrzynczak <twawrzynczak@chromium.org>
Change-Id: Ib3046233667e97a5f78961fabacbeb3099b3d442
---
M src/include/fw_config.h
M src/lib/fw_config.c
2 files changed, 15 insertions(+), 2 deletions(-)