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/usage.c

@@ -26,5 +26,5 @@ usage(void)
 	    util, util, util, util,
 	    util, util, util);
 
-	err_exit(EINVAL, "Too few arguments");
+	exitf("Too few arguments");
 }