"Mixin" layer for adding Rust 1.70.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 1.70.x toolchain for
Kirkstone by backporting the appropriate recipes from the nanbield
branch of openembedded-core.

Notes:
=======
- cargo-update-recipe-crates.bbclass has been backported with a tweak to
  use host rather than native Python since kirkstone does not have a new
  enough Python (3.11 is required).  Using the update task in the class
  will fail unless the host environment contains Python 3.11 or newer.
- The newer version of librsvg from nanbield 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.
- 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 upgrades that occur in the
  nanbield 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>