bootc: support grub2 serial console customization

Wire the blueprint bootloader customization into the bootc disk image
pipeline.
When blueprint specifies grub2 serial/terminal settings,
emit an org.osbuild.grub2.d stage that writes a drop-in config to
`boot/grub2/console.cfg` after bootc install-to-filesystem.

This relies on `bootupd`'s `console.cfg` grub config: https://github.com/coreos/bootupd/blob/main/src%2Fgrub2%2Fgrub-static-pre.cfg#L41

This adds a Go wrapper for the new osbuild stage and reads
`GetBootloader()` in the bootc image type.

Assisted-by: Opencode.ai <Opus 4.6>
Requires: osbuild/blueprint#53
Requires: osbuild/osbuild#2473

Author: jbtrystram

data/distrodefs/bootc-generic/imagetypes.yaml

@@ -87,6 +87,7 @@
 
   supported_options_lists:
     supported_options_disk: &supported_options_disk
+      - "customizations.bootloader"
       - "customizations.directories"
       - "customizations.disk"
       - "customizations.files"

pkg/distro/generic/bootc_imagetype.go

@@ -256,6 +256,20 @@ func (t *bootcImageType) manifestForDisk(bp *blueprint.Blueprint, options distro
 		img.OSCustomizations.KernelOptionsAppend = append(img.OSCustomizations.KernelOptionsAppend, kopts.Append)
 	}
 
+	if bl := customizations.GetBootloader(); bl != nil && bl.Grub2 != nil {
+		grub2Cfg := &osbuild.GRUB2Config{}
+		if len(bl.Grub2.TerminalInput) > 0 {
+			grub2Cfg.TerminalInput = bl.Grub2.TerminalInput
+		}
+		if len(bl.Grub2.TerminalOutput) > 0 {
+			grub2Cfg.TerminalOutput = bl.Grub2.TerminalOutput
+		}
+		if bl.Grub2.Serial != nil {
+			grub2Cfg.Serial = *bl.Grub2.Serial
+		}
+		img.OSCustomizations.Grub2Config = grub2Cfg
+	}
+
 	rootfsMinSize := max(bd.rootfsMinSize, options.Size)
 
 	pt, err := t.genPartitionTable(customizations, rootfsMinSize, rng)

pkg/manifest/raw_bootc.go

@@ -309,6 +309,22 @@ func (p *RawBootcImage) serialize() (osbuild.Pipeline, error) {
 			postStages = append(postStages, ignitionStage)
 		}
 
+		// Apply grub2 console configuration (terminal_input, terminal_output,
+		// serial) via the grub2.d stage which writes a drop-in config file
+		// under boot/grub2/. Like ignition, this writes to /boot so we use
+		// bootupd mounts.
+		if grub2dCfg := osbuild.NewGrub2DConfigFromGrub2Config(p.OSCustomizations.Grub2Config); grub2dCfg != nil {
+			grub2dOpts := &osbuild.Grub2DStageOptions{
+				Config: grub2dCfg,
+			}
+			grub2dStage := osbuild.NewGrub2DStage(grub2dOpts)
+			grub2dStage.Devices, grub2dStage.Mounts, err = osbuild.GenBootupdDevicesMounts(p.filename, p.PartitionTable, p.platform)
+			if err != nil {
+				return osbuild.Pipeline{}, fmt.Errorf("gen devices for grub2.d stage failed %w", err)
+			}
+			postStages = append(postStages, grub2dStage)
+		}
+
 		pipeline.AddStages(postStages...)
 
 		// In case we created any files in the deploy directory we need to relabel

pkg/osbuild/grub2_d_stage.go

@@ -0,0 +1,50 @@
+package osbuild
+
+// Grub2DStageOptions represents options for the
+// org.osbuild.grub2.d stage.
+//
+// This stage writes a GRUB2 drop-in configuration file at a
+// configurable path relative to the filesystem root. The path
+// defaults to "boot/grub2/console.cfg".
+type Grub2DStageOptions struct {
+	// Path relative to the filesystem root
+	// (default: "boot/grub2/console.cfg")
+	Path string `json:"path,omitempty"`
+
+	// GRUB2 configuration to write
+	Config *Grub2DConfig `json:"config"`
+}
+
+// Grub2DConfig contains GRUB2 settings for a drop-in config file.
+type Grub2DConfig struct {
+	TerminalInput  []string `json:"terminal_input,omitempty"`
+	TerminalOutput []string `json:"terminal_output,omitempty"`
+	Serial         string   `json:"serial,omitempty"`
+}
+
+func (Grub2DStageOptions) isStageOptions() {}
+
+func NewGrub2DStage(options *Grub2DStageOptions) *Stage {
+	return &Stage{
+		Type:    "org.osbuild.grub2.d",
+		Options: options,
+	}
+}
+
+// NewGrub2DConfigFromGrub2Config creates a Grub2DConfig from a
+// GRUB2Config, extracting only the console-related fields.
+// Returns nil if no console settings are present.
+func NewGrub2DConfigFromGrub2Config(cfg *GRUB2Config) *Grub2DConfig {
+	if cfg == nil {
+		return nil
+	}
+	c := &Grub2DConfig{
+		TerminalInput:  cfg.TerminalInput,
+		TerminalOutput: cfg.TerminalOutput,
+		Serial:         cfg.Serial,
+	}
+	if len(c.TerminalInput) == 0 && len(c.TerminalOutput) == 0 && c.Serial == "" {
+		return nil
+	}
+	return c
+}