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