hack to download olsrd tarball with more identifying filename

currently, due to Github's archive download URLs, olsrd is being downloaded as <branch name>.tar.gz, which is hardly identifying. It will help our build script if it is downloaded as olsrd-0.6.5.4-<branch name>.tar.gz, so we can better remove it from the dl directory when we want to pull in a fresh version.

to test, build olsrd for openwrt and make sure it succeeds. There should be a file in the `dl` folder called `olsrd-0.6.5.4-commotion.tar.gz`.