Introduce History= setting

Introduce History= setting

Currently, to boot an image with mkosi qemu after building it with
mkosi build, various settings have to be identical to when mkosi build
was invoked to make sure that mkosi can find the outputs of the previous
build. Because this is rather error prone and annoying, let's introduce
a History= setting to allow mkosi to remember the configuration of the
last build which can then be read again when running a verb that operates
on a built image.

Another case where this is extremely useful is when some part of the
configuration changes every single time, for example if mkosi.version
is an executable script that outputs the current time, which is then
encoded in the output name, we have to remember the previous config,
otherwise mkosi wouldn't be able to find the outputs of the previous
build.

Note that while we load the configuration of the previous build, we
ignore all settings from the [Host] section which we read again from
the configuration files, as the user should be able to change these
without rebuilding the image.