`mkosi summary`-esque helper function

[Winterhuman]

mkosi: v19

Description

Parsing /work/src/mkosi.conf within a chrooted script is relatively simple to do, however, when mkosi.conf.d/*.conf or mkosi.images are involved things could become much more complicated. Ideally, there would be a way to run mkosi summary from within chrooted scripts to get an overview of the overall effective config.

What I'm proposing is a mkosi summary-esque helper function which prints the full list of config options in an easy-to-parse format. As an example to demonstrate:

$ mkosi-summary-helper
Locale=en_GB.UTF-8
Distribution=arch
CompressOutput=zst
Packages=bash less linux sudo systemd
...

$ mkosi-summary-helper | grep "Locale" | ...

This helper function would use the same config parsing logic as mkosi itself, accounting for mkosi.conf.d, mkosi.images, etc, which would eliminate the complexity of reimplementing the parsing logic.

[DaanDeMeyer]

As discussed in matrix, we could probably just expose the JSON config for the current image as $MKOSI_CONFIG

[DaanDeMeyer]

$MKOSI_CONFIG is now set to a path containing the mkosi config serialized as JSON. See #2473