Update to mkosi v19

- Use mkosi.images/ instead of mkosi.presets/
- Use the .chroot suffix to run scripts in the image
- Use BuildSources= match for the kernel build
- Move 10-systemd.conf to mkosi.conf and rely on mkosi.local.conf
  for local configuration

.gitignore

@@ -34,9 +34,7 @@ __pycache__/
 /mkosi.installdir/
 /mkosi.key
 /mkosi.crt
-# Ignore any mkosi config files with "local" in the name
-/mkosi.conf.d/**/*local*.conf
-mkosi.conf
+mkosi.local.conf
 /tags
 .dir-locals-2.el
 .vscode/