Image Customizer: Support string mountPoint (#10862)

Author: Adub17030MS

toolkit/tools/imagecustomizer/docs/configuration.md

@@ -1010,6 +1010,25 @@ The meaning of this value depends on the type property.
 
 ## mountPoint type
 
+You can configure `mountPoint` in one of two ways:
+
+1. **Structured Format**: Use `idType`, `options`, and `path` fields for a more detailed configuration.
+   
+   ```yaml
+   mountPoint:
+     path: /boot/efi
+     options: umask=0077
+     idType: part-uuid
+   ```
+
+2. **Shorthand Path Format**: Provide the mount path directly as a string when only `path` is required.
+
+   ```yaml
+   mountPoint: /boot/efi
+   ```
+
+   In this shorthand format, only the `path` is specified, and default values will be applied to any optional fields.
+
 ### idType [string]
 
 Default: `part-uuid`

toolkit/tools/imagecustomizerapi/mountpoint.go

@@ -1,10 +1,10 @@
-// Copyright (c) Microsoft Corporation.
-// Licensed under the MIT License.
-
 package imagecustomizerapi
 
 import (
 	"fmt"
+
+	"github.com/microsoft/azurelinux/toolkit/tools/internal/sliceutils"
+	"gopkg.in/yaml.v3"
 )
 
 // MountPoint holds the mounting information for each partition.
@@ -17,6 +17,34 @@ type MountPoint struct {
 	Path string `yaml:"path"`
 }
 
+// UnmarshalYAML enables MountPoint to handle both a shorthand path and a structured object.
+func (p *MountPoint) UnmarshalYAML(value *yaml.Node) error {
+	// Check if the node is a scalar (i.e., single path string).
+	if value.Kind == yaml.ScalarNode {
+		// Treat scalar value as the Path directly.
+		p.Path = value.Value
+		return nil
+	}
+
+	// yaml.Node.Decode() doesn't respect the KnownFields() option.
+	// So, manually enforce this.
+	validFields := []string{"idType", "options", "path"}
+	for i := 0; i < len(value.Content); i += 2 {
+		key := value.Content[i].Value
+		if !sliceutils.ContainsValue(validFields, key) {
+			return fmt.Errorf("line %d: field %s not found in type %s", value.Line, key, "MountPoint")
+		}
+	}
+
+	// Otherwise, decode as a full MountPoint struct.
+	type IntermediateTypeMountPoint MountPoint
+	err := value.Decode((*IntermediateTypeMountPoint)(p))
+	if err != nil {
+		return fmt.Errorf("failed to parse MountPoint struct:\n%w", err)
+	}
+	return nil
+}
+
 // IsValid returns an error if the MountPoint is not valid
 func (p *MountPoint) IsValid() error {
 	err := p.IdType.IsValid()