automatically use "mkosi.extra" in the current directory as extra subdir
automatically use "mkosi.extra" in the current directory as extra subdir

With this mkosi.default in the local dir may contain config data,
mkosi.nspawn may contain nspawn settings data and mkosi.extra may
contain extra drop-in files for the image.

systemd/mkosi,systemd/mkosi,UbiCastTeam/mkosi,UbiCastTeam/mkosi