Make builds a bit more robust and reusable

- Lock our vendored dependencies (also pull in some new ones I will be
needing soon)

- Ensure we have a sane way of pulling those vendored dependencies into
the DFU-flashable images (upy-freeze.txt)

- Break up the Makefile into a bunch of smaller, sometimes reusable
chunks

- Add a noop keyboard that simply exits, so we can flash a "keyboard"
that just dumps us straight to REPL

- Remove my personal ampy config since that's not portable at all

upy-freeze.txt

@@ -0,0 +1,4 @@
+vendor/upy-lib/__future__/__future__.py
+vendor/upy-lib/functools/functools.py
+vendor/upy-lib/string/string.py
+vendor/pydux/pydux