rust: Enable building rust from stable, beta and nightly channels

Rust follows the train release model via the stable, beta and nightly channels,
by default we build rust from the stable channel, however there are certain
features which are only available in the beta or nightly channels.

Make these channels available by setting a RUST_CHANNEL variable which defaults
to stable making this change transparent to the user.

The snapshot version used by rust during its compilation wont necessarily match
the version being built, specially if were building from an unstable channel,
to avoid confusion rename this to SNAPSHOT_VERSION and use RUST_VERSION for the
version to be built, which is automatically defined to PV.

Append -beta or -nightly to rusts PV for signature awareness.

It is important to note that this does not build rust from the beta/nightly
published tarball (which today build rust v1.67.0 and v1.68.0 respectively),
instead this builds rust from the current selected version (1.66.0) and enables
the beta/nightly features for that version.

Setting the variable RUST_CHANNEL=nightly results in the following:

$ rustc -Vv
rustc 1.66.0-nightly

Signed-off-by: Alejandro Hernandez Samaniego <alejandro@enedino.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>