osbuild: add ostree.deployment mount type

New mount type added: org.osbuild.ostree.deployment.

Adding this to a stage will setup all needed bind mounts so that a given
`tree` will look like an active OSTree deployment, very much as OSTree
does during early boot.

This is often necessary when making changes to files in /etc for an
ostree image.

internal/osbuild/ostree_deployment_mount.go

@@ -0,0 +1,32 @@
+package osbuild
+
+type OSTreeMountOptions struct {
+	Deployment OSTreeMountDeployment `json:"deployment"`
+}
+
+func (OSTreeMountOptions) isMountOptions() {}
+
+type OSTreeMountDeployment struct {
+	// Name of the stateroot to be used in the deployment
+	OSName string `json:"osname"`
+
+	// OStree ref to create and use for deployment
+	Ref string `json:"ref"`
+
+	// The deployment serial (usually '0')
+	Serial *int `json:"serial,omitempty"`
+}
+
+func NewOSTreeDeploymentMount(name, osName, ref string, serial int) *Mount {
+	return &Mount{
+		Type: "org.osbuild.ostree.deployment",
+		Name: name,
+		Options: &OSTreeMountOptions{
+			Deployment: OSTreeMountDeployment{
+				OSName: osName,
+				Ref:    ref,
+				Serial: &serial,
+			},
+		},
+	}
+}