efi_loader: Pass fdt address directly to bootefi cmd

The bootefi cmd today fetches its device tree pointer from either the
location appointed by "fdt addr" with a fallback to the U-Boot control
fdt.

This integration is unusual for U-Boot and diverges from the way we
usually handle parameters to boot commands. So let's pass the fdt
directly into the bootefi command instead and move the control fdt
logic into the distro boot script.

Signed-off-by: Alexander Graf <agraf@suse.de>
Acked-by: Stephen Warren <swarren@wwwdotorg.org>
Reviewed-by: Andreas Färber <afaerber@suse.de>