Extract rust version to a variable

This will make it easier to update in the future.

toolset/setup/linux/languages/rust.sh

@@ -1,14 +1,16 @@
 #!/bin/bash
 
+RUST_VERSION="1.7.0"
+
 RETCODE=$(fw_exists $IROOT/rust.installed)
 [ ! "$RETCODE" == 0 ] || { \
   source $IROOT/rust.installed;
   return 0; }
 
-fw_get -O https://static.rust-lang.org/dist/rust-1.7.0-x86_64-unknown-linux-gnu.tar.gz
-fw_untar rust-1.7.0-x86_64-unknown-linux-gnu.tar.gz
+fw_get -O https://static.rust-lang.org/dist/rust-${RUST_VERSION}-x86_64-unknown-linux-gnu.tar.gz
+fw_untar rust-${RUST_VERSION}-x86_64-unknown-linux-gnu.tar.gz
 (
-	cd rust-1.7.0-x86_64-unknown-linux-gnu
+	cd rust-${RUST_VERSION}-x86_64-unknown-linux-gnu
 	./install.sh --prefix=$IROOT/rust
 )