bump to Rust 1.20.0 and Cargo 0.21

(cherry picked from commit 3bf7088579)

recipes-devtools/rust/rust-source-1.20.0.inc

@@ -0,0 +1,14 @@
+# Specifics for Rust 1.20.0
+
+SRC_URI += "\
+	https://static.rust-lang.org/dist/rustc-${PV}-src.tar.gz;name=rust \
+    "
+SRC_URI[rust.md5sum] = "1d3c5d25d8e6215e7d0b6d4d4c9835b9"
+SRC_URI[rust.sha256sum] = "2aa4875ff4472c6e35262bbb9052cb2623da3dae6084a858cc59d36f33f18214"
+
+# later versions of rust change the directory that they unextract to
+RUSTSRC = "${WORKDIR}/rustc-${PV}-src"
+# set this as our default
+S = "${RUSTSRC}"
+
+LIC_FILES_CHKSUM = "file://COPYRIGHT;md5=12922f5565a22267bd82aaeb6d3548e5"