Make install-crosstool-ng.sh script easy to rerun

Delete the previous build artifacts when rerunning the script instead of
failing. This won't happen in CI, but does in local debugging.

Author: imphil

install-crosstool-ng.sh

@@ -13,6 +13,7 @@ set -e
 
 mkdir -p "${build_top_dir}/build"
 
+rm -rf "${build_top_dir}/build/crosstool-ng"
 git clone https://github.com/lowRISC/crosstool-ng "${build_top_dir}/build/crosstool-ng"
 cd "${build_top_dir}/build/crosstool-ng"
 git checkout --force "${CROSSTOOL_NG_VERSION}"