This change will not do much for regular builds, but it allows to put the
firmware blob in a specific flash area.

In cases where multiple firmware versions run on the same device, this allows
re-use of the firmware blob. On such case is picowota, the Raspberry Pi Pico W
OTA flash bootloader. Until now, picowota users effectively have the firmware
on their devices twice.

Another use case is that in a firmware update, the CYW firmware can be left
untouched.

The pointer to the blob is put in a struct that also contains some metadata,
which is put in a separate section to allow linker script writers to prepend
the binary blob with something like a header.

To store the cyw firmware at a particular location in flash, these changes
can be made to the linker script:

MEMORY {
...
FLASH_FW_CYW(rx):            ORIGIN = 0x10040000, LENGTH = 256k
...
}

and

SECTIONS {
...
    .fw_cyw43 : {
        . = ALIGN(4k);
        KEEP(*(.fw_cyw43.meta))
        . = ALIGN(512);
        *(.fw_cyw43.blob)
    } > FLASH_FW_CYW
...
}