makefile: add fetch-variant

[jpculp]

Issue number:

Closes #176

Description of changes:

Adds a target to pull a pre-built variant using tuftool to the typical build directory. Supports both 'unified' and 'split' partitioned variants.

Testing done:

• [x] Downloaded artifacts with default root.json.
• [x] Downloaded artifacts with local root.json.
• [x] Attempted to download artifacts with a non-empty output directory.
• [x] Validated tuftool command when FETCH_VARIANT_UNIFIED=true.

Terms of contribution:

By submitting this pull request, I agree that this contribution is dual-licensed under the terms of both the Apache License, version 2.0, and the MIT license.