[grubcfg] Introduce prefer_grub_d

- new setting for using /etc/defaults/grub.d/ (SEE #1457), not implemented
- add missing fields to schema for config file

src/modules/grubcfg/grubcfg.conf

@@ -17,6 +17,11 @@
 # already existed. If set to false, edits the existing file instead.
 overwrite: false
 
+# If set to true, prefer to write files in /etc/default/grub.d/
+# rather than the single file /etc/default/grub. If this is set,
+# Calamares will write /etc/default/grub.d/00Calamares instead.
+prefer_grub_d: false
+
 # If set to true, an **existing** setting for GRUB_DISTRIBUTOR is
 # kept, not updated to the *bootloaderEntryName* from the branding file.
 # Use this if the GRUB_DISTRIBUTOR setting in the file is "smart" in

src/modules/grubcfg/grubcfg.schema.yaml

@@ -5,6 +5,8 @@ additionalProperties: false
 type: object
 properties:
     overwrite: { type: boolean, default: false }
+    keepDistributor: { type: boolean, default: false }
+    prefer_grub_d: { type: boolean, default: false }
     defaults:
         type: object
         additionalProperties: true  # Other fields are acceptable