bootm/fdt: Only process the fdt if an fdt address was provided

bootm/fdt: Only process the fdt if an fdt address was provided

Boards with CONFIG_OF_LIBFDT enabled are not able to boot old-style
kernels using the board info structure (instead of passing a device tree)
This change allows the old style booting to be used if the fdt argument
was not passed to 'bootm'.

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Acked-by: Kim Phillips <kim.phillips@freescale.com>