"Mixin" layer for adding current Rust 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 current Rust toolchain for Kirkstone
by backporting the appropriate recipes from the master 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 master 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.
- While changes to Rust recipe and class files related to oe-selftest
  support are included by necessity, no effort has been made to backport
  the actual addition of Rust to oe-selftest, as that is considered too
  invasive and maintenance heavy.
- To include Rust support in a SDK, do the following in e.g. local.conf:

  TOOLCHAIN_HOST_TASK:append = " packagegroup-rust-cross-canadian-${MACHINE}"
  TOOLCHAIN_TARGET_TASK:append = " ${@multilib_pkg_extend(d, 'libstd-rs')}"

  Overriding populate_sdk_base.bbclass to avoid needing to do this seems
  too invasive for this layer.
- The intent is to track any further upgrades that occur in the master
  branch of oe-core, with an expected end of support in April 2026, the
  same as Yocto Project's planned EOL for Kirkstone (per
  https://wiki.yoctoproject.org/wiki/Releases).

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