Print `FQBN` in verbose compile output (#2134)

* Added `FQBN.Clone()` method

* Slightly increased unti-test isolation

* Added `PackageManager.NormalizeFQBN()` method.

* Print FQBN in verbose compile output

arduino/cores/fqbn.go

@@ -76,6 +76,16 @@ func (fqbn *FQBN) String() string {
 	return res
 }
 
+// Clone returns a copy of this FQBN.
+func (fqbn *FQBN) Clone() *FQBN {
+	return &FQBN{
+		Package:      fqbn.Package,
+		PlatformArch: fqbn.PlatformArch,
+		BoardID:      fqbn.BoardID,
+		Configs:      fqbn.Configs.Clone(),
+	}
+}
+
 // Match check if the target FQBN corresponds to the receiver one.
 // The core parts are checked for exact equality while board options are loosely
 // matched: the set of boards options of the target must be fully contained within

arduino/cores/packagemanager/package_manager.go

@@ -827,3 +827,26 @@ func (pme *Explorer) FindMonitorDependency(discovery *cores.MonitorDependency) *
 		return toolRelease.GetLatestInstalled()
 	}
 }
+
+// NormalizeFQBN return a normalized copy of the given FQBN, that is the same
+// FQBN but with the unneeded or invalid options removed.
+func (pme *Explorer) NormalizeFQBN(fqbn *cores.FQBN) (*cores.FQBN, error) {
+	_, _, board, _, _, err := pme.ResolveFQBN(fqbn)
+	if err != nil {
+		return nil, err
+	}
+	normalizedFqbn := fqbn.Clone()
+	for _, option := range fqbn.Configs.Keys() {
+		values := board.GetConfigOptionValues(option)
+		if values == nil || values.Size() == 0 {
+			normalizedFqbn.Configs.Remove(option)
+			continue
+		}
+
+		defaultValue := values.Keys()[0]
+		if fqbn.Configs.Get(option) == defaultValue {
+			normalizedFqbn.Configs.Remove(option)
+		}
+	}
+	return normalizedFqbn, nil
+}

legacy/builder/target_board_resolver.go

@@ -36,9 +36,18 @@ func (s *TargetBoardResolver) Run(ctx *types.Context) error {
 			core = "arduino"
 		}
 		// select the core name in case of "package:core" format
+		normalizedFQBN, err := ctx.PackageManager.NormalizeFQBN(ctx.FQBN)
+		if err != nil {
+			ctx.Warn(fmt.Sprintf("Could not normalize FQBN: %s", err))
+			normalizedFQBN = ctx.FQBN
+		}
+		ctx.Info(fmt.Sprintf("FQBN: %s", normalizedFQBN))
+
 		core = core[strings.Index(core, ":")+1:]
 		ctx.Info(tr("Using board '%[1]s' from platform in folder: %[2]s", targetBoard.BoardID, targetPlatform.InstallDir))
 		ctx.Info(tr("Using core '%[1]s' from platform in folder: %[2]s", core, buildPlatform.InstallDir))
+
+		ctx.Info("")
 	}
 
 	if buildProperties.Get("build.board") == "" {