otk-gen-partition-table: add bootable flag support

This commit adds support to mark partitions as bootable from
an `otk-gen-partition-table` input. E.g.:
```yaml
{
...
  "partitions": [
    {
...
      "bootable": true,
...
    },
```

cmd/otk-gen-partition-table/main.go

@@ -51,6 +51,7 @@ type InputCreate struct {
 // InputPartition represents a single user provided partition input
 type InputPartition struct {
 	Name       string `json:"name"`
+	Bootable   bool   `json:"bootable"`
 	Mountpoint string `json:"mountpoint"`
 	Label      string `json:"label"`
 	Size       string `json:"size"`
@@ -184,9 +185,10 @@ func makePartitionTableFromOtkInput(input *Input) (*disk.PartitionTable, error)
 			return nil, err
 		}
 		pt.Partitions = append(pt.Partitions, disk.Partition{
-			Size: uintSize,
-			UUID: part.PartUUID,
-			Type: part.PartType,
+			Size:     uintSize,
+			UUID:     part.PartUUID,
+			Type:     part.PartType,
+			Bootable: part.Bootable,
 			// XXX: support lvm,luks here
 			Payload: &disk.Filesystem{
 				Label:        part.Label,

cmd/otk-gen-partition-table/main_test.go

@@ -16,6 +16,8 @@ import (
 )
 
 // see https://github.com/achilleas-k/images/pull/2#issuecomment-2136025471
+// Note that this partition table contains all json keys for testing, some may
+// contradict each other
 var partInputsComplete = `
 {
   "properties": {
@@ -32,6 +34,7 @@ var partInputsComplete = `
   "partitions": [
     {
       "name": "root",
+      "bootable": true,
       "mountpoint": "/",
       "label": "root",
       "size": "7 GiB",
@@ -69,6 +72,7 @@ var expectedInput = &genpart.Input{
 	Partitions: []*genpart.InputPartition{
 		{
 			Name:       "root",
+			Bootable:   true,
 			Mountpoint: "/",
 			Label:      "root",
 			Size:       "7 GiB",
@@ -307,6 +311,26 @@ func TestIntegrationRealistic(t *testing.T) {
 	assert.Equal(t, expectedSimplePartOutput, outp.String())
 }
 
+func TestGenPartitionTableBootable(t *testing.T) {
+	inp := &genpart.Input{
+		Properties: genpart.InputProperties{
+			Type: "dos",
+		},
+		Partitions: []*genpart.InputPartition{
+			{
+				Bootable:   true,
+				Mountpoint: "/",
+				Size:       "10 GiB",
+				Type:       "ext4",
+			},
+		},
+	}
+
+	output, err := genpart.GenPartitionTable(inp, rand.New(rand.NewSource(0))) /* #nosec G404 */
+	assert.NoError(t, err)
+	assert.Equal(t, true, output.Const.Internal.PartitionTable.Partitions[0].Bootable)
+}
+
 func TestGenPartitionTableMinimal(t *testing.T) {
 	// XXX: think about what the smalltest inputs can be and validate
 	// that it's complete and/or provide defaults (e.g. for "type" for