[health checker] Skip serial reboot for NUCs

NUCs can't seem to accept serial properly, so just skip serial
reboots. This should save ~4 minutes a run.

Change-Id: I88059200c8dff7ee3b84a08d68be891cfbabf790
diff --git a/devices/power.go b/devices/power.go
index 80f7c9b..cfb538f 100644
--- a/devices/power.go
+++ b/devices/power.go
@@ -72,6 +72,10 @@
 	var rebooter Rebooter
 	log.Printf("Attempting to soft reboot device %s", nodename)
 	if serial != nil {
+		// TODO(fxb/42761): Remove this short circuit once NUCs can accept serial properly
+		if c.Type == "AMT" {
+			return c.Powercycle(nodename, "", serial)
+		}
 		log.Printf("Using serial to reboot.")
 		rebooter = NewSerialRebooter(serial)
 	} else {