kernel-fitimage: fix external dtb check

kernel-fitimage: fix external dtb check

If EXTERNAL_KERNEL_DEVICETREE and dtb_image_sect are empty variables
dtb_path ends up as "/" which is available on most Unix systems but
probably not the dtb_path which is needed here. Checking for a file
makes more sense and also solves the issue with the "/".

Signed-off-by: Adrian Freihofer <adrian.freihofer@siemens.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>