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>

tools/mkimage.c

@@ -732,6 +732,12 @@ copy_file (int ifd, const char *datafile, int pad)
 		exit (EXIT_FAILURE);
 	}
 
+	if (sbuf.st_size == 0) {
+		fprintf (stderr, "%s: Input file %s is empty, bailing out\n",
+			params.cmdname, datafile);
+		exit (EXIT_FAILURE);
+	}
+
 	ptr = mmap(0, sbuf.st_size, PROT_READ, MAP_SHARED, dfd, 0);
 	if (ptr == MAP_FAILED) {
 		fprintf (stderr, "%s: Can't read %s: %s\n",