configs: meson64: add alternate USB DFU boot target

Add boot over DFU RAM as an alternate to running script at
a fixed address like done today.

The main culprit is that it's not possible to do that
on G12A/Sm1 platforms due to changes in the USB boot protocol.

With this, U-Boot will present a DFU device with a ram slot where
the Host could write a fitImage or legacy U-Boot image, then with the
detach command boot will continue trying to boot the uploaded image.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Link: https://lore.kernel.org/r/20231023-usb-dfu-boot-v1-3-df9d121c67c1@linaro.org


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