Add RUST_TARGET variable
Add RUST_TARGET environment variable through AC_ARG_VAR to allow the user to override the rust target name. Indeed, using $host when cross-compiling is not always the good option especially when vendor part of target is not set to unknown but to another value such as buildroot. Indeed, in this case aarch64-buildroot-linux-gnu won't be recognised as a valid target by rust/cargo.
Signed-off-by: Fabrice Fontaine firstname.lastname@example.org