SMBIOS: Implement SKU field

Leave it for the platform to fill in the string.

Change-Id: I7b4fe585f8d1efc8c9743f0d8b38de1f98124aab
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: https://review.coreboot.org/14996
Tested-by: build bot (Jenkins)
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-by: Philipp Deppenwiese <zaolin.daisuki@googlemail.com>

src/include/smbios.h

@@ -35,6 +35,7 @@ const char *smbios_mainboard_serial_number(void);
 const char *smbios_mainboard_version(void);
 void smbios_mainboard_set_uuid(u8 *uuid);
 const char *smbios_mainboard_bios_version(void);
+const char *smbios_mainboard_sku(void);
 u8 smbios_mainboard_enclosure_type(void);
 #ifdef CONFIG_MAINBOARD_FAMILY
 const char *smbios_mainboard_family(void);