Now that we have the mkosi.clangd script to run clangd from the mkosi
build script, it becomes clear that doing cleanup with mkosi.clean has
a big gap in that we always run the mkosi.clean script and thus we also
run it when we run the mkosi.clangd script, causing the previously built
packages to be removed when we run clangd without producing new ones.
In mkosi we're improving the situation by only running clean scripts when we
clean up the output directory and disallowing writing to the output directory
from build scripts.
Let's adapt systemd to these changes by moving the copying of packages to the
output directory to the postinst script.
In https://github.com/systemd/mkosi/pull/2847, the '@' specifier is
removed, CLI arguments take priority over configuration files again
and the "main" image is defined at the top level instead of in
mkosi.images/. Additionally, not every setting from the top level
configuration is inherited by the images in mkosi.images/ anymore,
only settings which make sense to be inherited are inherited.
This commit gets rid of all the usages of '@', moves the "main" image
configuration from mkosi.images/system to the top level and gets rid
of various hacks we had in place to deal with quirks of the old
configuration parsing logic.
We also remove usages of Images= and --append as these options are
removed by the mentioned PR.
