This adds a full eMMC image including U-Boot, which means that the
kernel can inherit the true RAM size detected by the preloader.
As implemented in previous commits, sysupgrade to this image from
the legacy layout (and via that, from the vendor-installed image)
is supported.
Rename the legacy image for the 512MiB board, for clarity.
Signed-off-by: David Woodhouse <dwmw2@infradead.org>
