rockboxdev: better error message for unknown compression

I hit this error and its message confused me because I interpreted it as
"bzip2 executable could not be found". The message was also outdated
because xz is supported meanwhile. Update the message to something
hopefully more helpful.

Change-Id: Ide2f8809dc0f8159d7d04fa22c29607dd6b35792

tools/rockboxdev.sh

@@ -271,7 +271,7 @@ extract() {
     elif [ -f "$dlwhere/$1.tar.xz" ]; then
         tar xJf "$dlwhere/$1.tar.xz"
     else
-        echo "ROCKBOXDEV: I don't know how to extract $1 (no bzip2 or gzip)"
+        echo "ROCKBOXDEV: unknown compression for $1"
         exit
     fi
 }