efi_loader: Use system fdt as fallback

When the user did not pass any device tree or the boot script
didn't find any, let's use the system device tree as last resort
to get something the payload (Linux) may understand.

This means that on systems that use the same device tree for U-Boot
and Linux we can just share it and there's no need to manually provide
a device tree in the target image.

While at it, also copy and pad the device tree by 64kb to give us
space for modifications.

Signed-off-by: Alexander Graf <agraf@suse.de>
Tested-by: Andreas Färber <afaerber@suse.de>