libreboot-utils: fix ALL compiler warnings

i wasn't using strict mode enough in make:

make strict

now it compiles cleanly. mostly removing
unused variables, fixing implicit conversions,
etc.

Signed-off-by: Leah Rowe <leah@libreboot.org>

util/libreboot-utils/lib/checksum.c

@@ -59,10 +59,10 @@ read_checksums(void)
 	if (_num_invalid >= _max_invalid) {
 
 		if (_max_invalid == 1)
-			err_exit(ECANCELED, "%s: part %lu has a bad checksum",
+			exitf("%s: part %lu has a bad checksum",
 			    f->fname, (size_t)f->part);
 
-		err_exit(ECANCELED, "%s: No valid checksum found in file",
+		exitf("%s: No valid checksum found in file",
 		    f->fname);
 	}
 }