Implement remote reboot.

Also fixes the issues around Core 0 restarting but Core 1 and DMA and everything else remaining running. We do this using the watchdog scratch registers.

Also cleans up the stack painting code and moves some functions around so they are all together.

Author: jonathanpallant

Cargo.lock

@@ -94,17 +94,26 @@ The Neotron BIOS uses the [defmt](https://crates.io/crates/defmt) crate to provi
    cargo install probe-run
    ```
 
-5. Power on your Neotron Pico by applying 12V and pressing the On/Off button.
-
-6. Build the Neotron OS:
+5. Build the Neotron OS:
     We use the "neotron-flash-1002.ld" linker script to link it at `0x1002_0000`.
 
     ```console
     user@host ~/neotron-os $ cargo build --bin=flash1002 --release --target=thumbv6m-none-eabi
     user@host ~/neotron-os $ arm-none-eabi-objcopy -O binary ./target/thumbv6m-none-eabi/release/flash1002 ../neotron-pico-bios/src/thumbv6m-none-eabi-flash1002-libneotron_os.bin
     ```
 
-7. Build and load the Neotron BIOS, and view the debug output stream, with `cargo run --release`:
+5. Power on your Neotron Pico into bootloader mode by applying 12V, holding down
+   the "BOOTSEL" button on the Raspberry Pi Pico and then and tapping the On/Off
+   button on the Neotron.
+
+   If you don't put the RP2040 into bootloader mode then when `probe-run` resets
+   the chip after programming, the firmware will detect it has had an incomplete
+   reset and cause a full reset. This makes the video output more reliable, but
+   the full reset will immediately disconnect `probe-run` so you won't see any
+   log messages. Booting the RP2040 in USB bootloader mode avoids this issue by
+   making the `probe-run` triggered reset look more like a full reset.
+
+7. Build and load the Neotron BIOS, and view the debug output stream, with `cargo run --release`.
 
     ```console
     user@host ~/neotron-pico-bios $ DEFMT_LOG=debug cargo run --release