util/sconfig: Probe device when fw_config is unprovisioned

When fw_config is unprovisioned (eg. in the factory), devices that do
not have any probe list are enabled by default and those that have probe
list are disabled. On mainboards that support multiple types of boot
critical devices (eg. storage) through probing fw_config, all of
them are disabled when fw_config is unprovisioned. Hence the devices do
not boot to OS. Add sconfig fw_config rule `probe unprovisioned` to
enable such devices when fw_config is unprovisioned.

BUG=None
TEST=Build Brox firmware and boot to OS when fw_config is unprovisioned.

Change-Id: I178f821e077912776d654971924d67203a7c43df
Signed-off-by: Karthikeyan Ramasubramanian <kramasub@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/83983
Reviewed-by: Jon Murphy <jpmurphy@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Eric Lai <ericllai@google.com>
Reviewed-by: Ren Kuo <ren.kuo@quanta.corp-partner.google.com>

src/include/device/device.h

@@ -150,6 +150,7 @@ struct device {
 
 	/* Zero-terminated array of fields and options to probe. */
 	DEVTREE_CONST struct fw_config *probe_list;
+	bool enable_on_unprovisioned_fw_config;
 };
 
 /**