spl: fit: Print error message when FDT is not present

When FDT is not present in the image user doesn't get any error what's
wrong. Print error message if LIBCOMMON_SUPPORT is enabled.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Seris-cc: uboot
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>