tcmode-default: add rust to the default toolchains

Remove the rust_versions.inc file and put it's content
in tcmode-default.inc is done for other toolchains.

Rename RUST_VERSION to RUSTVERSION to adhere to the
naming format used for other languages.

Signed-off-by: Randy MacLeod <Randy.MacLeod@windriver.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>