mb/google/rex/var/rex0: Use FW_CONFIG to select the correct SAR table

This patch changes the SAR table selection logic to use FW_CONFIG which
will eventually help to support different WiFi SAR tables.

TEST=Able to build and boot google/rex.

Change-Id: I8f1244e3c3715bc3fbe6be1ade87817ff19836de
Signed-off-by: YH Lin <yueherngl@google.com>
Signed-off-by: Subrata Banik <subratabanik@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/77428
Reviewed-by: Eric Lai <eric_lai@quanta.corp-partner.google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/mainboard/google/rex/Kconfig

@@ -48,6 +48,7 @@ config BOARD_GOOGLE_BASEBOARD_REX
 config BOARD_GOOGLE_MODEL_REX
 	def_bool n
 	select BOARD_GOOGLE_BASEBOARD_REX
+	select CHROMEOS_WIFI_SAR if CHROMEOS
 	select DRIVERS_GENERIC_MAX98357A
 	select DRIVERS_GENESYSLOGIC_GL9755
 	select DRIVERS_INTEL_ISH

src/mainboard/google/rex/variants/rex0/variant.c

@@ -12,7 +12,7 @@
 
 const char *get_wifi_sar_cbfs_filename(void)
 {
-	return "wifi_sar_0.hex";
+	return get_wifi_sar_fw_config_filename(FW_CONFIG_FIELD(WIFI));
 }
 
 void variant_generate_s0ix_hook(enum s0ix_entry entry)