Support legacy multi-type images without FDT section.

This patch enables legacy multi-type images containing only a Linux kernel
and root file system to be loaded, maintaining compatibility with previous
versions of u-boot.

This is required when using old image files such as a Linux 2.4 kernel /
filesystem.

Signed-off-by: Nick Spence <nick.spence@freescale.com>
Acked-by: Bartlomiej Sieka <tur@semihalf.com>