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

.gitmodules

@@ -1,3 +1,12 @@
 [submodule "circuitpython"]
 	path = vendor/circuitpython
 	url = https://github.com/adafruit/circuitpython.git
+	branch = "9b98ad779468676c3d5f1efdc06b454aaed7c407"
+[submodule "pydux"]
+	path = vendor/pydux
+	url = https://github.com/usrlocalben/pydux.git
+	branch = "943ca1c75357b9289f55f17ff2d997a66a3313a4"
+[submodule "upy-lib"]
+	path = vendor/upy-lib
+	url = https://github.com/micropython/micropython-lib.git
+	branch = "f20d89c6aad9443a696561ca2a01f7ef0c8fb302"