[job] Allow jobs to report an arbitrary status message

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/include/ipxe/job.h

 	 * account before calculating @c completed/total.
 	 */
 	unsigned long total;
+	/** Message (optional) */
+	char message[32];
 };
 
 extern int job_progress ( struct interface *intf,