[grubcfg] Keep kernel_params as 'quiet' by default

- mark some TODO
- tighten up the YAML schema a little bit
- when unset, use 'quiet' rather than empty, to preserve compatibility
  with existing configuration files.

SEE #1882

src/modules/grubcfg/grubcfg.conf

@@ -29,10 +29,14 @@ prefer_grub_d: false
 # kept, not updated to the *bootloaderEntryName* from the branding file.
 # Use this if the GRUB_DISTRIBUTOR setting in the file is "smart" in
 # some way (e.g. uses shell-command substitution).
+#
+# TODO:3.3:snake-case this key
 keepDistributor: false
 
-# The default kernel params that should always be applied
-# This is an array of strings
+# The default kernel params that should always be applied.
+# This is an array of strings. If it is unset, the default is
+# `["quiet"]`. To avoid the default, explicitly set this key
+# to an empty list, `[]`.
 kernel_params: [ "quiet" ]
 
 # Default entries to write to /etc/default/grub if it does not exist yet or if

src/modules/grubcfg/grubcfg.schema.yaml

@@ -7,9 +7,10 @@ additionalProperties: false
 type: object
 properties:
     overwrite: { type: boolean, default: false }
+    # TODO:3.3:snake-case this key
     keepDistributor: { type: boolean, default: false }
     prefer_grub_d: { type: boolean, default: false }
-    kernel_params: { type: array }
+    kernel_params: { type: array, items: { type: string } }
     defaults:
         type: object
         additionalProperties: true  # Other fields are acceptable

src/modules/grubcfg/main.py

@@ -172,7 +172,7 @@ def modify_grub_default(partitions, root_mount_point, distributor):
             if partition["fs"] == "zfs" and partition["mountPoint"] == "/":
                 zfs_root_path = get_zfs_root()
 
-    kernel_params = libcalamares.job.configuration.get("kernel_params", [])
+    kernel_params = libcalamares.job.configuration.get("kernel_params", ["quiet"])
 
     # Currently, grub doesn't detect this properly so it must be set manually
     if zfs_root_path: