PICO_UNIQUE_BOARD_ID_SIZE_BYTES macro

\file pico/unique_id.h Unique device ID access API RP2040 does not have an on-board unique identifier (all instances of RP2040 silicon are identical and have no persistent state). However, RP2040 boots from serial NOR flash devices which have a 64-bit unique ID as a standard feature, and there is a 1:1 association between RP2040 and flash, so this is suitable for use as a unique identifier for an RP2040-based board. This library injects a call to the flash_get_unique_id function from the hardware_flash library, to run before main, and stores the result in a static location which can safely be accessed at any time via pico_get_unique_id(). This avoids some pitfalls of the hardware_flash API, which requires any flash-resident interrupt routines to be disabled when called into.