stm32: implement reboot wait-ext

This was missed on stm32, but is helpful for servod
to work reliably.

BUG=b:77830536
TEST=it waits 10 sec for external reboot.

Change-Id: Ic4c905846c41b43f3b8542d70e021744716bd0c2
Signed-off-by: Nick Sanders <nsanders@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/1004437
Reviewed-by: Aseda Aboagye <aaboagye@chromium.org>
Reviewed-by: Scott Collyer <scollyer@chromium.org>

chip/stm32/system.c

@@ -398,6 +398,15 @@ void system_reset(int flags)
 		while (1)
 			;
 	} else {
+		if (flags & SYSTEM_RESET_WAIT_EXT) {
+			int i;
+
+			/* Wait 10 seconds for external reset */
+			for (i = 0; i < 1000; i++) {
+				watchdog_reload();
+				udelay(10000);
+			}
+		}
 		CPU_NVIC_APINT = 0x05fa0004;
 	}