fdt: fix extend of cmd line

On arm CONFIG_CMDLINE_EXTEND does not append build-in
cmdline in kernel to U-boot parameters. Fix it here.
Theoretically this patch should repair kdump work where
it adds elfcorehdr= and memmap additional parameters
to second kernel.

Signed-off-by: Max Uvarov <muvarov@gmail.com>
Signed-off-by: Rob Herring <robh@kernel.org>