stages: allow bootloader --append via kickstart

This commit allows use to append kernel commandline options via
the kickstart file. This is useful for e.g. the bootc installer
where we support customizing this via blueprints.

stages/org.osbuild.kickstart

@@ -243,6 +243,9 @@ def main(tree, options):  # pylint: disable=too-many-branches
     if reboot:
         config += [reboot]
     config += make_network(options)
+    kargs_append = options.get("bootloader", {}).get("append")
+    if kargs_append:
+        config += [f"bootloader --append='{kargs_append}'"]
 
     target = os.path.join(tree, path)
     base = os.path.dirname(target)

stages/org.osbuild.kickstart.meta.json

@@ -542,6 +542,17 @@
         "items": {
           "$ref": "#/definitions/network-options-ref"
         }
+      },
+      "bootloader": {
+        "description": "Specifies how the boot loader should be installed",
+        "additionalProperties": false,
+        "type": "object",
+        "properties": {
+          "append": {
+            "description": "Specifies additional kernel parameters",
+            "type": "string"
+          }
+        }
       }
     }
   }

stages/test/test_kickstart.py

@@ -234,6 +234,7 @@ TEST_INPUT = [
         },
         "ostreecontainer --url=http://some-ostree-url.com/foo",
     ),
+    ({"bootloader": {"append": "karg1 karg2=0"}}, "bootloader --append='karg1 karg2=0'"),
 ]