* Add pico_get_unique_board_id_string API
Add a new API to pico_unique which will turn the unique ID into a
canonical text string.
Use this API to update the USB device serial number in stdio_usb.
Supercedes #280
* Clean up -Wconversion=error issues
* Address review comments, fix api typing
Use cleaner binary-to-hex conversion. Update the length parameter to
use uint per the SDK standard .
- setting baud rate to magic value (default=1200) will cause a reset to BOOTSEL mode
- a VENDOR interface along side the CDC interface can be used to reset via refular flash boot, or into
BOOTSEL mode with control for the reset_usb_boot parameters for the latter
either method can be configured/enabled/disabled via #define
