internal/rosa/package: migrate system image

The overlay argument also enables migration of stage0.

Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/rosa/state.go

@@ -932,6 +932,7 @@ func (ctx *evalContext) pf(
 		output  pkg.Artifact
 		sourceA any
 		helper  Helper
+		overlay string
 
 		inputs, runtimes, extra azalea.Array
 	)
@@ -960,6 +961,7 @@ func (ctx *evalContext) pf(
 		k("inputs"):   &inputs,
 		k("runtime"):  &runtimes,
 		k("extra"):    &extra,
+		k("overlay"):  &overlay,
 	}); err != nil {
 		return
 	}
@@ -991,13 +993,26 @@ func (ctx *evalContext) pf(
 		})
 	}
 
-	for _, h := range extraH {
-		_, a := ctx.t.MustLoad(h)
-		attr.Paths = append(attr.Paths, pkg.Path(
-			AbsUsrSrc.Append("extra", h.String()),
-			attr.Writable,
-			a,
-		))
+	if overlay == "" {
+		for _, h := range extraH {
+			_, a := ctx.t.MustLoad(h)
+			attr.Paths = append(attr.Paths, pkg.Path(
+				AbsUsrSrc.Append("extra", h.String()),
+				attr.Writable,
+				a,
+			))
+		}
+	} else {
+		var o pkg.ExecPath
+		if o.P, err = check.NewAbs(overlay); err != nil {
+			o.P = AbsUsrSrc.Append("extra", overlay)
+		}
+		o.A = make([]pkg.Artifact, len(extraH))
+		for i, h := range extraH {
+			_, a := ctx.t.MustLoad(h)
+			o.A[i] = a
+		}
+		attr.Paths = append(attr.Paths, o)
 	}
 
 	for _, pair := range files {