blueprint: add support for selecting partition table type

Users need to have control over the partition table type in certain
cases. Most prominently, they need a dos-style partition table for
single-board computers.

Let's add a simple customizations for it.

pkg/blueprint/disk_customizations.go

@@ -14,13 +14,15 @@ import (
 )
 
 type DiskCustomization struct {
-	// TODO: Add partition table type: gpt or dos
+	// Type of the partition table: gpt or dos.
+	// Optional, the default depends on the distro and image type.
+	Type       string
 	MinSize    uint64
 	Partitions []PartitionCustomization
 }
 
 type diskCustomizationMarshaler struct {
-	// TODO: Add partition table type: gpt or dos
+	Type       string                   `json:"type,omitempty" toml:"type,omitempty"`
 	MinSize    datasizes.Size           `json:"minsize,omitempty" toml:"minsize,omitempty"`
 	Partitions []PartitionCustomization `json:"partitions,omitempty" toml:"partitions,omitempty"`
 }
@@ -30,6 +32,7 @@ func (dc *DiskCustomization) UnmarshalJSON(data []byte) error {
 	if err := json.Unmarshal(data, &dcm); err != nil {
 		return err
 	}
+	dc.Type = dcm.Type
 	dc.MinSize = dcm.MinSize.Uint64()
 	dc.Partitions = dcm.Partitions
 
@@ -345,6 +348,12 @@ func (p *DiskCustomization) Validate() error {
 		return nil
 	}
 
+	switch p.Type {
+	case "gpt", "dos", "":
+	default:
+		return fmt.Errorf("unknown partition table type: %s (valid: gpt, dos)", p.Type)
+	}
+
 	mountpoints := make(map[string]bool)
 	vgnames := make(map[string]bool)
 	var errs []error

pkg/blueprint/disk_customizations_test.go

@@ -904,6 +904,22 @@ func TestPartitioningValidation(t *testing.T) {
 			},
 			expectedMsg: "invalid partitioning customizations:\nmountpoint for swap logical volume with name \"swappylv\" in volume group \"badvg\" must be empty",
 		},
+		"gpt": {
+			partitioning: &blueprint.DiskCustomization{
+				Type: "gpt",
+			},
+		},
+		"dos": {
+			partitioning: &blueprint.DiskCustomization{
+				Type: "dos",
+			},
+		},
+		"unhappy-badtype": {
+			partitioning: &blueprint.DiskCustomization{
+				Type: "toucan",
+			},
+			expectedMsg: "unknown partition table type: toucan (valid: gpt, dos)",
+		},
 	}
 
 	for name := range testCases {
@@ -1737,6 +1753,15 @@ func TestDiskCustomizationUnmarshalJSON(t *testing.T) {
 				MinSize: 1 * datasizes.GiB,
 			},
 		},
+		"type": {
+			inputJSON: `{
+				"type": "gpt"
+			}`,
+			inputTOML: `type = "gpt"`,
+			expected: &blueprint.DiskCustomization{
+				Type: "gpt",
+			},
+		},
 	}
 
 	for name := range testCases {