Add section attributes to combined firmware blobs and metadata

[mvds00]

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
...
}

(edit: renamed flash memory and section to FLASH_FW_CYW and .fw_cyw43)

[peterharperuk]

Looks ok to me and a quick test suggests it doesn't break anything. We'd just have to be careful when updating the firmware that we don't break this, but I guess that's not a big issue.

Do you not care about the Bluetooth firmware? I guess it's fairly small.

[mvds00]

As I only use w43439A0_7_95_49_00_combined I was only able to test that, and modified anything that closely resembled it. For bluetooth the issue seems less severe as it is small indeed. Apparently I missed a new firmware header (w4343WA1_7_45_98_102_combined.h) which was not part of my (local) commit, so I amended the commit.