optional external repos?

can we have the tool point to existing repos / versions? say i have a linux kernel repo already and want to use that. same goes for coreboot and u-root. maybe the u-root can be done with a go.mod override.

on a related note, can we specify where these get dropped? it looks like the coreboot and linux repos get dropped in your current directory.

No files, tests, or entry points are named. Start by mapping how coreboot and Linux repositories are currently fetched, how versions are selected, and where repositories are written; then assess whether a u-root go.mod override fits. Done means users can reuse pinned external repositories and choose their destination without breaking image builds.

Written by the indexing model from the issue text.