um: Add os_warn() for pre-boot warning/error messages

Add os_warn() for printing out pre-boot warning/error
messages in stderr. The messages via os_warn() are not
suppressed by quiet option.

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

arch/um/os-Linux/util.c

@@ -177,3 +177,12 @@ void os_info(const char *fmt, ...)
 	vfprintf(stderr, fmt, list);
 	va_end(list);
 }
+
+void os_warn(const char *fmt, ...)
+{
+	va_list list;
+
+	va_start(list, fmt);
+	vfprintf(stderr, fmt, list);
+	va_end(list);
+}