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.

(cherry picked from commit 20345a86)