* 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 .
In C, func() is a function taking an unspecified number of arguments,
vs func(void) a function taking no arguments. In C++ both forms indicate
"no arguments."
Update these headers to use the (void) form, which is correct in both
languages and avoids complaints when -Wstrict-prototypes is specified.
