Fix missing MTD error handling - add specific error

Summary:
Currently, upgrades that fail because of a blank /proc/mtd are being classed as `UNSAFE_TO_REBOOT` errors. This is because to classify as a `BAD_FLASH_CHIP` error, Flashy is grepping for the particular string *'Cannot access MTD device'* whereas the error on the BMC is returning:
```
root@rsw006-oob:~# cat /proc/mtd
dev:    size   erasesize  name
root@rsw006-oob:~# fw_printenv bootargs
Cannot open /dev/mtd1: No such file or directory
```
Let's fix this by skipping the string grepping logic completely, as we already know that we cannot see MTD at this stage

Also added as separate error class, as missing MTD is a specific issue that we see quite frequently and good to classify it separately

Test Plan:
Unit tests
```
$ ./tools/flashy/scripts/run_unit_tests.sh
...
...
...
142/142 unit tests passed
```
 ---
Will handle exceptions in oobgrader and test in following diff

Reviewed By: doranand

Differential Revision: D60115096

fbshipit-source-id: 77e0cb6aa18fdb71e4c711d3951c7282aab23b6a

Author: cjcon90

tools/flashy/checks_and_remediations/common/04_ensure_flash_available.go

@@ -75,17 +75,13 @@ func ensureFlashAvailable(stepParams step.StepParams) step.StepExitError {
 	}
 
 	// Check for mtdparts aleady being set.
-	cmd = []string{"fw_printenv", "bootargs"}
+	cmd = []string{"ls", "/dev/mtd*"}
 	_, err, stdout, stderr = utils.RunCommand(cmd, 30*time.Second)
 	if err != nil {
-		if strings.Contains(stderr, "Cannot access MTD device") {
-			errMsg := errors.Errorf(
-				"U-Boot environment is inaccessible: broken flash chip?" +
-				" Error code: %v, stderr: %v", err, stderr)
-			return step.ExitBadFlashChip{Err: errMsg}
-		}
-		log.Printf("fw_printenv doesn't work: %v, stderr: %v", err, stderr)
-		return nil
+		errMsg := errors.Errorf(
+			"Broken flash chip? U-Boot environment is inaccessible." +
+			" Error code: %v, stderr: %v", err, stderr)
+		return step.ExitMissingMtd{Err: errMsg}
 	}
 
 	// Override mtdparts on next boot.

tools/flashy/checks_and_remediations/common/04_ensure_flash_available_test.go

@@ -103,26 +103,15 @@ func TestEnsureFlashAvailable(t *testing.T) {
 			want:              step.ExitMustReboot{Err: errors.Errorf("Forcing reboot for new bootargs to take effect")},
 		},
 		{
-			name:              "fw_printenv broken because reasons",
-			vbootUtilExists:   false,
-			lfOpenBMC:         false,
-			failGrep:          false,
-			failPrint:         true,
-			failSet:           false,
-			printOutput:       "",
-			grepOutput:        "",
-			want:              nil,
-		},
-		{
-			name:              "fw_printenv broken because missing flash",
+			name:              "missing flash",
 			vbootUtilExists:   false,
 			lfOpenBMC:         false,
 			failGrep:          false,
 			failPrint:         true,
 			failSet:           false,
 			printOutput:       "Cannot access MTD device /dev/mtd1: No such file or directory",
 			grepOutput:        "",
-			want:              step.ExitBadFlashChip{Err: errors.Errorf("U-Boot environment is inaccessible: broken flash chip? Error code: err1, stderr: Cannot access MTD device /dev/mtd1: No such file or directory")},
+			want:              step.ExitMissingMtd{Err: errors.Errorf("Broken flash chip? U-Boot environment is inaccessible. Error code: err1, stderr: Cannot access MTD device /dev/mtd1: No such file or directory")},
 		},
 		{
 			name:              "fw_setenv broken",
@@ -149,7 +138,7 @@ func TestEnsureFlashAvailable(t *testing.T) {
 					} else {
 						return 0, nil, tc.grepOutput, ""
 					}
-				} else if (cmdArr[0] == "fw_printenv") {
+				} else if (cmdArr[0] == "ls") {
 					if (tc.failPrint) {
 						return 1, errors.Errorf("err1"), "", tc.printOutput
 					} else {

tools/flashy/lib/step/error.go

@@ -36,6 +36,7 @@ const (
 	FLASHY_ERROR_SAFE_TO_REBOOT   = 42
 	FLASHY_ERROR_UNSAFE_TO_REBOOT = 52
 	FLASHY_ERROR_BROKEN_SYSTEM    = 53
+	FLASHY_ERROR_MISSING_MTD      = 54
 )
 
 type StepExitError interface {
@@ -56,6 +57,10 @@ type ExitBadFlashChip struct {
 	Err error
 }
 
+type ExitMissingMtd struct {
+	Err error
+}
+
 type ExitMustReboot struct {
 	Err error
 }
@@ -100,6 +105,18 @@ func (e ExitBadFlashChip) GetType() string {
 	return "ExitBadFlashChip"
 }
 
+func (e ExitMissingMtd) GetError() string {
+	return e.Err.Error()
+}
+
+func (e ExitMissingMtd) GetExitCode() int {
+	return FLASHY_ERROR_MISSING_MTD
+}
+
+func (e ExitMissingMtd) GetType() string {
+	return "ExitMissingMtd"
+}
+
 func (e ExitMustReboot) GetError() string {
 	return e.Err.Error()
 }
@@ -157,6 +174,7 @@ func HandleStepError(err StepExitError) {
 	case ExitUnsafeToReboot,
 		ExitUnknownError,
 		ExitBadFlashChip,
+		ExitMissingMtd,
 		ExitMustReboot:
 
 		encodeExitError(err)