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/drivers/serial.c

@@ -195,7 +195,7 @@ int serial_getchar(void)
 /* A vt100 doesn't do color, setaf/setab below are from xterm-color. */
 #define VT100_SET_COLOR   "\e[3%d;4%dm"
 
-static void serial_putcmd(char *str)
+static void serial_putcmd(const char *str)
 {
 	while(*str)
 		serial_putchar(*(str++));