tool: migrate to new environment info gathering

[zaklogician]

As part of a series of patches which upstream Viper export to Microkit, we separate SDK environment discovery from main.rs into sdk.rs.

SDK discovery is the following process:

1. finding the location of the Microkit directory based on the MICROKIT_SDK env variable, or if that's not set then based on the location of the tool executable itself)
2. finding the list of available boards and configs (debug/release/etc.)

This has to happen before argument parsing, and lets us give better error messages (e.g. we can report available configs if an invalid --config argument is given).

[zaklogician]

This is ready for review, the failing check is a wontfix gitlint issue resulting from my long email address.