zephyr: flash_area_get_device_id implementation broken for ARM devices

[de-nordic]

There is a bug where flash_area_get_device_id will return flash area partition id instead of device id when compiled on ARM arch.

This happens because code returns flash_area.fa_id, where it should fa_device_id. The fa_device_id member does not exist in Zephyr variant of Flash Area API defined flash_area type.

This device id is only used by flash_device_base function, which is provided by system backend, to return offset where device FLASH to RAM mapping begins.

Current implementation of flash_device_base:

mcuboot/boot/zephyr/flash_map_extended.c

Lines 43 to 52 in c6f5db2

	int flash_device_base(uint8_t fd_id, uintptr_t *ret)
	{
	if (fd_id != FLASH_DEVICE_ID) {
	BOOT_LOG_ERR("invalid flash ID %d; expected %d",
	fd_id, FLASH_DEVICE_ID);
	return -EINVAL;
	}
	*ret = FLASH_DEVICE_BASE;
	return 0;
	}

will fail to return for ID that is different than FLASH_DEVICE_ID, which will, for Zephyr, be hardcoded as 0 or 1, to identify internal and external memory respectively, and Flash Area device ID may be anything from 0 to 255.

Bug has been introduced with #1661

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