"Mixin" layer for adding Rust 1.68.x toolchain into the Yocto Project LTS.

At the time Kirkstone was released in May 2022 it included Rust 1.59.0,
and officially Kirkstone supports only that. This thin special-purpose
mixin layer is meant to provide a working Rust 1.68.x toolchain for
Kirkstone by backporting the appropriate recipes from the mickledore
branch of openembedded-core.

Notes:
- cargo-update-recipe-crates.bbclass has been left out of the backport
  since it would bump the Python requirement to 3.10, and that is a
  bigger can of worms than this mixin attempts to address.  It is
  recommended that recipe crates .inc files are updated using a newer
  YP release and backported.
- The newer version of librsvg from mickledore has been backported since
  there is not a straightforward way to update the existing one via
  bbappend.  Backporting a working recipe should be easier to maintain
  than developing an update of the older recipe in kirkstone.  There are
  no breaking API changes mentioned in the librsvg release notes between
  the two versions.
- python3-cryptography has been left alone since the recipe in kirkstone
  still works with the newer toolchain, and updating it seems to have
  more potential impact due to upstream API changes and its known to be
  finicky build.  This may change if sufficient rationale for doing the
  backport becomes apparent.
- The intent is to just track any further 1.68.x upgrades that occur in
  the mickledore branch of oe-core.  Supporting newer versions of Rust
  or trying to support more than one version of Rust should use a
  different mixin layer.

Maintainers:
Scott Murray <scott.murray@konsulko.com>
