S368275: don't mess with the watchdog on darwin

Summary:
Messing with a stopped watchdog starts it.

Can do something nicer later.

Test Plan:
```
0 ~/local/openbmc/tools/flashy $ go test ./...
?       github.com/facebook/openbmc/tools/flashy/flash_procedure        [no test files]
?       github.com/facebook/openbmc/tools/flashy/lib/logger     [no test files]
?       github.com/facebook/openbmc/tools/flashy/tests  [no test files]
?       github.com/facebook/openbmc/tools/flashy/utilities      [no test files]
ok      github.com/facebook/openbmc/tools/flashy        3.350s
ok      github.com/facebook/openbmc/tools/flashy/checks_and_remediations/bletchley      0.008s
ok      github.com/facebook/openbmc/tools/flashy/checks_and_remediations/common 0.296s
ok      github.com/facebook/openbmc/tools/flashy/checks_and_remediations/galaxy100      0.008s
ok      github.com/facebook/openbmc/tools/flashy/checks_and_remediations/wedge100       0.012s
ok      github.com/facebook/openbmc/tools/flashy/checks_and_remediations/yamp   0.009s
ok      github.com/facebook/openbmc/tools/flashy/install        0.009s
ok      github.com/facebook/openbmc/tools/flashy/lib/fileutils  0.014s
ok      github.com/facebook/openbmc/tools/flashy/lib/flash      0.011s
ok      github.com/facebook/openbmc/tools/flashy/lib/flash/flashcp      0.011s
ok      github.com/facebook/openbmc/tools/flashy/lib/flash/flashutils   0.008s
ok      github.com/facebook/openbmc/tools/flashy/lib/flash/flashutils/devices   0.009s
ok      github.com/facebook/openbmc/tools/flashy/lib/step       0.012s
ok      github.com/facebook/openbmc/tools/flashy/lib/utils      0.455s
ok      github.com/facebook/openbmc/tools/flashy/lib/validate   0.009s
ok      github.com/facebook/openbmc/tools/flashy/lib/validate/image     0.014s
ok      github.com/facebook/openbmc/tools/flashy/lib/validate/partition 0.037s
0 ~/local/openbmc/tools/flashy $ ./build.sh && ./build_dev.sh
0 ~/local/openbmc/tools/flashy $
```

```
$ oobgrader --host rsw029-oob.p076.f01.rva3.tfbnw.net --primary-only --wait --flashy-tag 734df50 --force --allow-downgrade
```
-> https://fburl.com/scuba/openbmc_upgrades/7e89e86b

Console messages during upgrade (no mention of watchdog):

```
[ 2783.001706] 11_drop_caches (6255): drop_caches: 3
[ 2873.352788] reboot: Restarting system

U-Boot 2019.04 fbdarwin-v2022.27.1 (Jul 06 2022 - 20:26:37 +0000)
```

Reviewed By: kawmarco

Differential Revision: D49734788

fbshipit-source-id: 1c2ce62b9d94f7fe8eb66a1969b0a2fc1d0c19a9

tools/flashy/lib/utils/system.go

@@ -365,6 +365,19 @@ var IsLFOpenBMC = func() (bool) {
 	return strings.Contains(string(osReleaseBuf), magic)
 }
 
+// IsBMCLite check whether the system is running BMC-lite
+// For S368275.   Make this beautiful later.
+var IsBMCLite = func() (bool) {
+	const magic = "fbdarwin"
+
+	issueBuf, err := fileutils.ReadFile(etcIssueFilePath)
+	if err != nil {
+		return false
+	}
+
+	return strings.Contains(string(issueBuf), magic)
+}
+
 // CheckOtherFlasherRunning return an error if any other flashers are running.
 // It takes in the baseNames of all flashy's steps (e.g. 00_truncate_logs)
 // to make sure no other instance of flashy is running.
@@ -519,11 +532,17 @@ func tryPetWatchdog() bool {
 //   are no concurrent instances of wdtcli, the watchdog timeout will be
 //   extended and the watchdog petted.
 var PetWatchdog = func() {
-    // LF-OpenBMC relies on systemd to pet the watchdog, so there is nothing
-    // to do here.
-    if IsLFOpenBMC() {
-        return
-    }
+	// LF-OpenBMC relies on systemd to pet the watchdog, so there is nothing
+	// to do here.
+	if IsLFOpenBMC() {
+		log.Printf("Watchdog not petted; LF OpenBMC")
+		return
+	}
+
+	if IsBMCLite() {
+		log.Printf("Watchdog not petted; BMC Lite")
+		return
+	}
 
 	for i := 0; i < 10; i++ {
 		if tryPetWatchdog() {

tools/flashy/lib/utils/system_test.go

@@ -724,6 +724,54 @@ func TestIsLFOpenBMC(t *testing.T) {
 	}
 }
 
+func TestIsBMCLite(t *testing.T) {
+	// mock and defer restore ReadFile
+	readFileOrig := fileutils.ReadFile
+	defer func() {
+		fileutils.ReadFile = readFileOrig
+	}()
+	cases := []struct {
+		name             string
+		readFileContents string
+		readFileError    error
+		want             bool
+	}{
+		{
+			name: "Is BMC Lite",
+			readFileContents: `OpenBMC Release fbdarwin-v2022.27.1`,
+			readFileError: nil,
+			want:          true,
+		},
+		{
+			name:             "Not BMC Lite",
+			readFileContents: `foobar`,
+			readFileError:    nil,
+			want:             false,
+		},
+		{
+			name:             "/etc/issue file read error",
+			readFileContents: "",
+			readFileError:    errors.Errorf("file read error"),
+			want:             false,
+		},
+	}
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			fileutils.ReadFile = func(filename string) ([]byte, error) {
+				if filename != "/etc/issue" {
+					t.Errorf("filename: want '%v' got '%v'",
+						"/etc/issue", filename)
+				}
+				return []byte(tc.readFileContents), tc.readFileError
+			}
+
+			got := IsBMCLite()
+			if tc.want != got {
+				t.Errorf("want '%v' got '%v'", tc.want, got)
+			}
+		})
+	}
+}
 
 func TestCheckOtherFlasherRunning(t *testing.T) {
 	// mock and defer restore