Reboot to USB mode on panic

On panic, the USB boot mode implemented in ROM will be called, providing access for UF2 uploads and picotool.

Usage

Just add this to your main.rs:

extern crate rp2040_panic_usb_boot;

Panic messages

Additionally, panic messages are written to the XIP RAM, after disabling XIP caching.

That way, the panic message can be read using picotool:

picotool save -r 0x15000000 0x15004000 message.bin
hexdump -C message.bin

The RAM contents can be read the same way, by reading from address 0x20000000.