Release 2020.6

One notable feature: ostree now supports `/` and `/boot` being on the same filesystem.
I know this has been a long time coming and often demanded - thanks to everyone
who contributed, but particularly @wmanley who even did two separate implementations to
better compare the advantages/disadvantages!
PR: https://github.com/ostreedev/ostree/pull/2149

Other than that it's mostly bugfixes; there is one quite important one for anyone using the `readonly=true` for `/sysroot` (which is still just Fedora CoreOS I suspect).

There's some improvements to the GObject Introspection metadata, some (cosmetic) static analyzer fixes, a fix for the immutable bit on s390x, dropping a deprecated bit in the systemd unit file, etc.

Thanks to everyone who contributed!

```
Colin Walters (11):
      Post-release version bump
      tests/inst: Bump to latest ostree and gtk-rs
      pull: Assign idle_src variable before calling unref()
      prepare-root: Remove unused variable
      admin/pin: Enforce that index is a number
      tests: Check the immutable bit
      linuxfsutil: Pass int to ioctl, not long
      tests/inst: Port to new sh-inline repo
      tests/inst: Update to published sh-inline crate
      Release 2020.6
      Post-release version bump

Felix Krull (2):
      lib: add some missing version tags
      lib: mark out parameters as out parameters

Jonathan Lebon (4):
      configure.ac: Set is_release_build=no
      ostree-prepare-root: Fix /etc bind mount
      ostree-remount: Remount /etc rw if needed
      ci: Temporarily import kola test from jlebon's FCOS fork

Matt Bilker (1):
      Fix mkinitcpio with newer systemd versions

Simon McVittie (1):
      boot: Replace deprecated StandardOutput=syslog with journal, etc.

William Manley (2):
      Refactor tests/bootloader-entries-crosscheck.py
      sysroot: Support /boot on root or as seperate filesystem for syslinux and u-boot

```

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