Maybe move shared bash scripts to Elixir?

[jjcarstens]

This is a general discussion based on work in #576. Specifically, to solve that issue we needed to get details from the top level firmware project being used and pass it down to a bash script stored in a cached nerves_system_* that is originally from and maintained by nerves_system_br.

Environment variable setting and passing is fine, but in going through it I wondered if some of these scripts would be better served through the Elixir tooling and shared there instead (which would also reduce requirement of bash).

I haven't done a full audit, but a definitely candidate would be the scripts/rel2fw.sh. Maybe there are more? Maybe this is a terrible idea? idk ...needs investigation