configs: meson64: declare addr out of EXTRA_ENV_SETTINGS

In order to reuse addresses for DFU RAM, define them separately,
it's cleaner and will be easier to override.

Link: https://lore.kernel.org/r/20231023-usb-dfu-boot-v1-2-df9d121c67c1@linaro.org


Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>