fw: Introduce an explicit state machine - changes protocol!

We introduce an explicit state machine (see README).

With the new states we:

- combine setting size and USS to a single command.
- start the device app immediatiely when having receceived the last
  data chunk and returning the digest.
- Loop forever and wait for the stick to be removed if we end up in
  unknown state.

Signed-off-by: Michael Cardell Widerkrantz <mc@tillitis.se>

doc/system_description/firmware.md

@@ -18,6 +18,54 @@ If your available `objcopy` and `size` commands is anything other than
 the default `llvm-objcopy` and `llvm-size` define `OBJCOPY` and `SIZE`
 to whatever they're called on your system.
 
+## Firmware state machine
+
+States:
+
+- `initial` - At start.
+- `init_loading` - Reset app digest, size, `USS` and load address.
+- `loading` - Expect more app data or a reset by `LoadApp()`.
+- `run` - Computes CDI and starts the device app.
+
+Commands in state `initial`:
+
+| *command*          | *next state*   |
+|--------------------|----------------|
+| NameVersion()      | unchanged      |
+| GetUDI()           | unchanged      |
+| LoadApp(size, uss) | `init_loading` |
+|                    |                |
+
+Commands in `init_loading`:
+
+| *command*          | *next state*   |
+|--------------------|----------------|
+| NameVersion()      | unchanged      |
+| GetUDI()           | unchanged      |
+| LoadApp(size, uss) | `init_loading` |
+| LoadAppData(data)  | `loading`      |
+|                    |                |
+
+Commands in `loading`:
+
+| *command*          | *next state*                     |
+|--------------------|----------------------------------|
+| NameVersion()      | unchanged                        |
+| GetUDI()           | unchanged                        |
+| LoadApp(size, uss) | `init_loading`                   |
+| LoadAppData(data)  | `loading` or `run` on last chunk |
+|                    |                                  |
+
+Behaviour:
+
+- NameVersion: identifies stick.
+- GetUDI: returns the Unique Device ID with vendor id, product id,
+  revision, serial number.
+- LoadApp(size, uss): Start loading an app with this size and user
+  supplied secret.
+- LoadAppData(data): Load chunk of data of app. When last data chunk
+  is received we compute and return the digest.
+
 ## Using QEMU
 
 Checkout the `tk1` branch of [our version of the