mkimage: clarify error message for empty input files

Currently, an empty imput file causes `mmap()` to fail, and you get an
error like "mkimage: Can't read file.img: Invalid argument", which is
extremely unintuitive and hard to diagnose if you don't know what to
look for. Add an explicit check for an empty file and provide a clear
error message instead.

We already bounds check the image size when listing and re-signing
existing images, so we only need this check here, when opening data
files going into a image.

Signed-off-by: Thomas Hebb <tommyhebb@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>