um: Use os_warn to print out pre-boot warning/error messages

Use os_warn() instead of printf/fprintf to print out
pre-boot warning/error messages to stderr.
Note that the help message and version message are
kept to print out to stdout, because user explicitly
specifies those options to get such information.

Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Richard Weinberger <richard@nod.at>

arch/um/os-Linux/execvp.c

@@ -136,7 +136,7 @@ int main(int argc, char**argv)
 	int ret;
 	argc--;
 	if (!argc) {
-		fprintf(stderr, "Not enough arguments\n");
+		os_warn("Not enough arguments\n");
 		return 1;
 	}
 	argv++;