Reduce warnings/errors in libpayload when using picky compiler options

The new build system uses quite a few more -W flags for the compiler by
default than the old one. And that's for the better.

Change-Id: Ia8e3d28fb35c56760c2bd0983046c7067e8c5dd6
Signed-off-by: Patrick Georgi <patrick@georgi-clan.de>
Reviewed-on: http://review.coreboot.org/72
Tested-by: build bot (Jenkins)
Reviewed-by: Uwe Hermann <uwe@hermann-uwe.de>

payloads/libpayload/libc/printf.c

@@ -114,7 +114,7 @@ static int printf_putstr(const char *str, struct printf_spec *ps)
 	size_t count;
 
 	if (str == NULL) {
-		char *nullstr = "(NULL)";
+		const char *nullstr = "(NULL)";
 		return printf_putnchars(nullstr, strlen(nullstr), ps);
 	}