Don't start PetWatchdogLoop on BMC Lite platforms

Summary: Just noticed we don't try petting the watchdog on BMC Lite platforms, replicate behaviour on `PetWatchdogLoop` and avoid problems similar to S368275

Test Plan:
Successfully upgrade fbdarwin device, ensure relevant log message appears:
```
 2025 Mar 21 12:53:47 rsw013-oob.p060.f01.ncg2.tfbnw.net kern.info fbdarwin-2024.50.0: [flashy|/run/flashy/flash_procedure/flash_fbdarwin --imagepath /run/upgrade/image --device mtd:flash0][3068]: 2025/03/21 12:53:47 Not starting PetWatchdogLoop; BMC Lite
```

Reviewed By: cjcon90

Differential Revision: D71643054

fbshipit-source-id: 814de38830348871122314859b25d65632b484b8

Author: kawmarco

tools/flashy/lib/utils/system.go

@@ -590,6 +590,7 @@ func tryPetWatchdog(wait bool) bool {
 //   - When /dev/watchdog it NOT busy and there are no concurrent instances of
 //     wdtcli, the watchdog timeout will be extended and the watchdog petted.
 var PetWatchdog = func() {
+	// S368275: Don't open /dev/watchdog on BMC Lite platforms
 	if IsBMCLite() {
 		log.Printf("Watchdog not petted; BMC Lite")
 		return
@@ -606,6 +607,12 @@ var PetWatchdog = func() {
 }
 
 var PetWatchdogLoop = func() {
+	// S368275: Don't open /dev/watchdog on BMC Lite platforms
+	if IsBMCLite() {
+		log.Printf("Not starting PetWatchdogLoop; BMC Lite")
+		return
+	}
+
 	log.Printf("PetWatchdogLoop started")
 	for {
 		if tryPetWatchdog(true) {