osbuild2/xorrisofs: make Boot section optional

Should be a pointer otherwise it creates an empty object in the manifest
which makes the stage fail.

Co-Developed-by: Christian Kellner <christian@kellner.me>

internal/distro/rhel84/distro_v2.go

@@ -780,7 +780,7 @@ func (t *imageTypeS2) xorrisofsStageOptions() *osbuild.XorrisofsStageOptions {
 		Filename: t.Filename(),
 		VolID:    fmt.Sprintf("RHEL-8-4-0-BaseOS-%s", t.Arch().Name()),
 		SysID:    "LINUX",
-		Boot: osbuild.XorrisofsBoot{
+		Boot: &osbuild.XorrisofsBoot{
 			Image:   "isolinux/isolinux.bin",
 			Catalog: "isolinux/boot.cat",
 		},

internal/distro/rhel90/stage_options.go

@@ -304,7 +304,7 @@ func xorrisofsStageOptions(filename string, arch string) *osbuild.XorrisofsStage
 		Filename: filename,
 		VolID:    fmt.Sprintf("RHEL-9-0-0-BaseOS-%s", arch),
 		SysID:    "LINUX",
-		Boot: osbuild.XorrisofsBoot{
+		Boot: &osbuild.XorrisofsBoot{
 			Image:   "isolinux/isolinux.bin",
 			Catalog: "isolinux/boot.cat",
 		},

internal/osbuild2/xorrisofs_stage.go

@@ -10,7 +10,7 @@ type XorrisofsStageOptions struct {
 	// System ID to set
 	SysID string `json:"sysid,omitempty"`
 
-	Boot XorrisofsBoot `json:"boot,omitempty"`
+	Boot *XorrisofsBoot `json:"boot,omitempty"`
 
 	EFI string `json:"efi,omitempty"`