cargo_common: Handle build SYS as well as HOST/TARGET

Improve the common class to handle BUILD_SYS as well as host and target,
removing the need to a workaround in the rust recipe.

(From OE-Core rev: 0dc9635c28c269f782ad4ed6ea06b4e4de1beb8e)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/recipes-devtools/rust/rust.inc

@@ -29,9 +29,6 @@ setup_cargo_environment () {
     # x86_64-unknown-linux-gnu.
     # Later stages are build for the native target (i.e. target.x86_64-linux)
     cargo_common_do_configure
-
-    printf '[target.%s]\n' "${RUST_BUILD_SYS}" >> ${CARGO_HOME}/config
-    printf "linker = '%s'\n" "${RUST_BUILD_CCLD}" >> ${CARGO_HOME}/config
 }
 
 inherit rust-target-config