Mention --sccache in build-toolchain usage message.

Author: rjmansfield

utils/build-toolchain

@@ -32,6 +32,9 @@ function usage() {
   echo "--distcc"
   echo "Build with distcc to speed up the toolchain build"
   echo ""
+  echo "--sccache"
+  echo "Build with sccache to speed up the toolchain build"
+  echo ""
   echo "--preset-file"
   echo "load build-script presets from the specified file"
   echo ""