[retry] Expose retry_poll() to explicitly poll all running timers

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

src/include/ipxe/retry.h

 extern void start_timer_fixed ( struct retry_timer *timer,
 				unsigned long timeout );
 extern void stop_timer ( struct retry_timer *timer );
+extern void retry_poll ( void );
 
 /**
  * Start timer with no delay

src/net/retry.c

 }
 
 /**
- * Single-step the retry timer list
+ * Poll the retry timer list
  *
- * @v process		Retry timer process
  */
-static void retry_step ( struct process *process __unused ) {
+void retry_poll ( void ) {
 	struct retry_timer *timer;
 	unsigned long now = currticks();
 	unsigned long used;
 	}
 }
 
+/**
+ * Single-step the retry timer list
+ *
+ * @v process		Retry timer process
+ */
+static void retry_step ( struct process *process __unused ) {
+	retry_poll();
+}
+
 /** Retry timer process */
 PERMANENT_PROCESS ( retry_process, retry_step );