This implements the new "yafti" launcher and ties everything together:
- The "/etc/profile.d" script is now a symlink to an immutable location, instead of an actual plaintext file, to prevent risk of user systems breaking due to ostree thinking that the script had been modified. We can now be certain that systems always work properly when switching between distros or uBlue flavors.
- Yafti is only installed if the "yafti" flag is enabled.
- If the user disables the "yafti" flag, we completely wipe out the entire yafti.yml configuration, all of our new "yafti autostart" components, and the "profile.d" symlink, so that nothing remains on the system image. If an upstream image installed yafti itself in `/usr/bin`, then we leave that component alone though (because "pip uninstall" and the potential errors it might throw seems like a pointless hassle just for a tiny, unused binary).
This new functionality now makes it possible to execute scripts at the start or end of the build process, while also being super simple to expand to add further script stages in the future.
It also supports effortless reuse of scripts for multiple stages, since the scripts are now executed with the "current stage" as their 1st argument, to allow them to easily determine which stage they're running in.
This fixes the issue where someone specifies `fedora-version: latest`, which won't be known until build-time.
I also added a small "welcome" banner to the build log. It's really just there to retain a somewhat contrived use-case example for how to use `get_yaml_string()`, for other programmers who want to extend this in the future.
You can now define your custom repos with the `%FEDORA_VERSION%` variable, to automatically use the correct repo version, so that you never have to maintain their custom URLs again in the future.
You can now easily remove RPMs from your custom image, without having to edit the build.sh script.
This also changes the old "rpms" config key, to "rpm-install", for consistency with the new setting.
Thanks to leveraging Bash arrays, we are now able to use parameter expansion to perform every package installation as one rpm-ostree operation instead of individual ones. This will give us a huge build speedup.
Bash properly expands every array element as safely quoted arguments, complete with support for spaces and special characters.
This new technique is way easier to use, more maintainable, and safely handles special characters and spaces in the YAML data. It will also enable other features, such as the rpm-ostree improvement coming in the next commit.
The function name is `get_yaml_array()` because it literally reads it into a Bash array, by the way. Don't change the function name in the future. We may need other functions to read values into strings, etc.
Lowercase, run-on sentences are a nightmare to expand later, because the lack of sentence structure means you can't just add more lines and have a nice flow anymore.
Let's add some punctuation and grammar.
* fix: prevent /etc/ merge issues
discussed at length in #28
also including tons of comments about the issue
* fix: change references of /etc/ to /usr/etc
