Use script for config downloads

[TheMrIron2]

Per previous PR #137 and branch config-refactor, use get-config-scripts.sh to get configs instead of manually wget from Savannah. Ensures overarching logic matches and has local fallbacks if Savannah is unreachable.

Cleaned up the noise and removed any orthogonal elements.

[zeldin]

LGTM. But just so you know: By placing get_config_scripts.sh in the scripts/ directory, you make it a "build script". Which means that running toolchain.sh without arguments will add this script to the list of things to run. But because of lexical ordering, it will be run after all other scripts, where it can do no good (although it will do no harm either).

[TheMrIron2]

I think we ought to move it somewhere more appropriate in that case. Beside the local copies of config, or do you have another suggestion?

[zeldin]

Well, having the three files in the same dir is fine, but then I don't think it should be called assets. Something like tools or utils would probably be appropriate for the script, and maybe config if you want a dir for all three files.

[clienthax]

Please wrap these for CI purposes

if [ -z "${NO_SAVANNAH}" ]; then
    fetch_config_file config.guess
    fetch_config_file config.sub
else
    echo "NO_SAVANNAH is set, skipping Savannah downloads."
    cp "${ASSET_DIR}/config.guess" "config.guess"
    cp "${ASSET_DIR}/config.sub" "config.sub"
fi

[TheMrIron2]

@zeldin Agreed with the suggestion. They're now grouped in the config directory.
I can add the NO_SAVANNAH flag if we agree on it, but I decided against incorporating it as part of the expected changes.