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/include/shared/os.h

@@ -244,6 +244,8 @@ extern void um_early_printk(const char *s, unsigned int n);
 extern void os_fix_helper_signals(void);
 extern void os_info(const char *fmt, ...)
 	__attribute__ ((format (printf, 1, 2)));
+extern void os_warn(const char *fmt, ...)
+	__attribute__ ((format (printf, 1, 2)));
 
 /* time.c */
 extern void os_idle_sleep(unsigned long long nsecs);