Proper config file overwriting in postmarketOS

With apk, it is possible to overwrite configuration files of other packages by using replaces in APKBUILD. I wrote Packaging: Override Configuration Files to explain it in detail and how the migration path from the currently used post-install approach will look like.

While working on this, I realized that we have too many config files to do the migration in one step. So I wrote two short scripts in a new postmarketos-mvcfg package (part of the first MR), which we can use to incrementally modernize the pmaports.

TODO:

• Modernize postmarketos-base.post-install and add postmarketos-mvcfg (!1713 (closed))
• Add /etc/os-release to postmarketos-base with new replaces logic (!1522 (closed))
• Adjust postmarketos-hidden-desktop-entries to use replaces instead of /etc/skel
• Migrate postmarketos-base.trigger (currently writes /etc/inittab depending on deviceinfo) (#866 (closed))
• postmarketos-ui-* packages: use tinydm, so we don't need to write the username at post-install time to the lightdm config (phosh: !1659 (closed))
• Modernize all device packages, which write to configs in post-install
  • device packages can override configs from other postmarketOS packages now!
• Modernize all other packages writing configs in post-install