nixos/system/activation/bootspec: adopt the latest specification

nixos/modules/system/activation/bootspec.cue

@@ -1,4 +1,6 @@
-#V1: {
+import "struct"
+
+#BootspecV1: {
 	system:         string
 	init:           string
 	initrd?:        string
@@ -7,12 +9,23 @@
 	kernelParams: [...string]
 	label:    string
 	toplevel: string
-	specialisation?: {
-		[=~"^"]: #V1
-	}
-	extensions?: {...}
 }
 
-Document: {
-	v1: #V1
+// A restricted document does not allow any official specialisation
+// information in it to avoid "recursive specialisations".
+#RestrictedDocument: struct.MinFields(1) & {
+	"org.nixos.bootspec.v1": #BootspecV1
+	[=~"^"]:                 #BootspecExtension
+}
+
+// Specialisations are a hashmap of strings
+#BootspecSpecialisationV1: [string]: #RestrictedDocument
+
+// Bootspec extensions are defined by the extension author.
+#BootspecExtension: {...}
+
+// A "full" document allows official specialisation information
+// in the top-level with a reserved namespaced key.
+Document: #RestrictedDocument & {
+	"org.nixos.specialisation.v1"?: #BootspecSpecialisationV1
 }