expose sysfs overlay option

This change allows to pass sysfs-overlay option required to generate a preseeding image which enhances the initial boot time of Ubuntu Core 20.

internal/commands/snap.go

@@ -15,6 +15,7 @@ type SnapOpts struct {
 	Snaps                     []string       `long:"snap" description:"Install extra snaps. These are passed through to \"snap prepare-image\". The snap argument can include additional information about the channel and/or risk with the following syntax: <snap>=<channel|risk>" value-name:"SNAP"`
 	CloudInit                 string         `long:"cloud-init" description:"cloud-config data to be copied to the image" value-name:"USER-DATA-FILE"`
 	Revisions                 map[string]int `long:"revision" description:"The revision of a specific snap to install in the image." value-name:"REVISION"`
+	SysfsOverlay              string         `long:"sysfs-overlay" description:"The optional sysfs overlay to used for preseeding. Directories from /sys/class/* and /sys/devices/platform will be bind-mounted to the chroot when preseeding"`
 }
 
 type SnapCommand struct {

internal/statemachine/snap_states.go

@@ -46,6 +46,7 @@ func (stateMachine *StateMachine) prepareImage() error {
 	// preseeding-related
 	imageOpts.Preseed = snapStateMachine.Opts.Preseed
 	imageOpts.PreseedSignKey = snapStateMachine.Opts.PreseedSignKey
+	imageOpts.SysfsOverlay = snapStateMachine.Opts.SysfsOverlay
 	imageOpts.AppArmorKernelFeaturesDir = snapStateMachine.Opts.AppArmorKernelFeaturesDir
 	imageOpts.SeedManifestPath = filepath.Join(stateMachine.commonFlags.OutputDir, "seed.manifest")