x86/smbios: Untangle system and board tables

We were used to set the same values in the system and board tables.
We'll keep the mainboard values as defaults for the system tables,
so nothing changes unless somebody overrides the system table hooks.

Change-Id: I3c9c95a1307529c3137647a161a698a4c3daa0ae
Signed-off-by: Nico Huber <nico.huber@secunet.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/29477
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Richard Spiegel <richard.spiegel@silverbackltd.com>
Reviewed-by: HAOUAS Elyes <ehaouas@noos.fr>

src/mainboard/google/reef/mainboard.c

@@ -88,7 +88,7 @@ void __weak variant_board_ec_set_skuid(void)
 {
 }
 
-const char *smbios_mainboard_sku(void)
+const char *smbios_system_sku(void)
 {
 	static char sku_str[7]; /* sku{0..255} */