define callback APIs for auxiliary firmware update.

TEST="COV=1 make" passes
	depthcharge still compiles
	in combination with follow-up CLs, ps8751 firmware update
	succeeds.

BUG=b:35586896

Change-Id: Ibadc41e56e4e25ee0aba5c83caa0e3596fb9ad20
Reviewed-on: https://chromium-review.googlesource.com/505259
Commit-Ready: Caveh Jalali <caveh@google.com>
Tested-by: Caveh Jalali <caveh@google.com>
Reviewed-by: Randall Spangler <rspangler@chromium.org>

firmware/stub/vboot_api_stub.c

@@ -187,6 +187,17 @@ VbError_t VbExEcBatteryCutOff(void)
 	return VBERROR_SUCCESS;
 }
 
+VbError_t VbExCheckAuxFw(VbAuxFwUpdateSeverity_t *severity)
+{
+	*severity = VB_AUX_FW_NO_UPDATE;
+	return VBERROR_SUCCESS;
+}
+
+VbError_t VbExUpdateAuxFw(void)
+{
+	return VBERROR_SUCCESS;
+}
+
 enum VbEcBootMode_t VbGetMode(void)
 {
 	return vboot_mode;