mb/purism/librem_skl: select DRIVERS_GENERIC_CBFS_SERIAL

This driver was previously added for another out-of-tree Librem device, but
forgot to switch over the librem_skl boards to use it. Remove
duplicate functionality from mainboard.c and delete the empty file.

Test: build/boot Librem 13v2 and verify serial number read from CBFS
via dmidecode.

Signed-off-by: Matt DeVillier <matt.devillier@puri.sm>
Change-Id: Ide952197335c6bfbad846c6d6f62be5c4c57e2cb
Reviewed-on: https://review.coreboot.org/c/coreboot/+/41040
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>
diff --git a/src/mainboard/purism/librem_skl/Kconfig b/src/mainboard/purism/librem_skl/Kconfig
index c1a9aec..9760a2f 100644
--- a/src/mainboard/purism/librem_skl/Kconfig
+++ b/src/mainboard/purism/librem_skl/Kconfig
@@ -1,6 +1,7 @@
 config BOARD_PURISM_BASEBOARD_LIBREM_SKL
 	def_bool n
 	select BOARD_ROMSIZE_KB_16384
+	select DRIVERS_GENERIC_CBFS_SERIAL
 	select HAVE_ACPI_RESUME
 	select HAVE_ACPI_TABLES
 	select INTEL_GMA_HAVE_VBT
diff --git a/src/mainboard/purism/librem_skl/mainboard.c b/src/mainboard/purism/librem_skl/mainboard.c
deleted file mode 100644
index eb28262..0000000
--- a/src/mainboard/purism/librem_skl/mainboard.c
+++ /dev/null
@@ -1,38 +0,0 @@
-/* SPDX-License-Identifier: GPL-2.0-only */
-/* This file is part of the coreboot project. */
-
-#include <smbios.h>
-#include <string.h>
-#include <cbfs.h>
-
-#define MAX_SERIAL_LENGTH 0x100
-
-const char *smbios_mainboard_serial_number(void)
-{
-	static char serial_number[MAX_SERIAL_LENGTH + 1] = {0};
-	struct cbfsf file;
-
-	if (serial_number[0] != 0)
-		return serial_number;
-
-	if (cbfs_boot_locate(&file, "serial_number", NULL) == 0) {
-		struct region_device cbfs_region;
-		size_t serial_len;
-
-		cbfs_file_data(&cbfs_region, &file);
-
-		serial_len = region_device_sz(&cbfs_region);
-		if (serial_len <= MAX_SERIAL_LENGTH) {
-			if (rdev_readat(&cbfs_region, serial_number, 0,
-					serial_len) == serial_len) {
-				serial_number[serial_len] = 0;
-				return serial_number;
-			}
-		}
-	}
-
-	strncpy(serial_number, CONFIG_MAINBOARD_SERIAL_NUMBER,
-			MAX_SERIAL_LENGTH);
-
-	return serial_number;
-}